CA2635037A1

CA2635037A1 - Holographic photochromic recording material, its preparation, and photonic devices fabricated thereof with enhanced thermal stability

Info

Publication number: CA2635037A1
Application number: CA002635037A
Authority: CA
Inventors: Yuri B. Boiko
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-06-18
Filing date: 2008-06-18
Publication date: 2008-10-01

Abstract

Holographic photochromic recording material, which is based on the photochromic dyes with attached unsaturated double bonds, such as those with the structure (see formula I) where at least one of R1 and R2 or both are -O(CH2) n OOCC(M)=CH2 with n=1, 2, 3, ...and M being -H or -CH3 while the remaining R i (or none) is selected from the series -OH, -OCH3, -OC2H5, -OC4H9, -OC5H11 ; ... -OC n H2n+1 (i={1,2); n=0, 1, 2, 3, 4, 5, ... ), and whose unsaturated double bonds are capable of participating in the chain reaction of co-polymerization with (met)-acryl ate monomers/oligomers.
After preparation, which involves dissolving all the ingredients, the material allows sample fabrication via filling in the cuvette and conducting thermally induced polymerization under 100 °C. In turn, the sample allows fabrication of holographic devices via photochromic recording of holograms, with enhanced thermal stability of the recorded holograms. Specifically, long term stability of the holograms is ensured at temperature not less then 100 °C.

Description

Description Summary Significant enhancement of the thermal stability of the hologram recorded on photochromic materials had been achieved via covalent bonding of the photochromic dye to the polymer matrix as compared to the host-guest systems. One time partial reduction of the hologram's initial diffractiori efficiency due to thermal exposure was observed for both - polymers with attached dye as well as host-guest materials. Such one time reduction is interpreted to be due to the thermal relaxation of the polymer network induced with photochromic transition in the dye molecule. A gradual hologram erasure at elevated temperatures was observed for the host-guest system, which is assumed to be due to dye's diffusion between highly lit and dark areas. For the photochromic materials with covalent boding of the dye to the polymer matrix the level of thermal stability is demonstrated such that at temperatures ca. 100 C there was no detectable diffusion type degradation of the hologram after 6 hours of exposure to elevated temperature.
Same heating of the hologram in photochromic host-guest polymer led to a full hologram erasure within 40 minutes. In both cases the one time initial DE reduction due to polymer matrix relaxation at elevated temperatures had comparable value of about 0.5 of hologram's initial DE.

Key words: photochromic, hologram, volume hologram, thermochromism, diarylethene, dye doped polymer, host-guest, thermal stability.
1. Introduction Photochromic hologram's thermal stability has importance for practical applications of those recording materials in optical storage [1-3], photonics waveguide [4] and grating devices, particularly when technical conditions assume that device operation may not be affected by extended exposure to elevated temperatures.
Resistance of the photochromic dye itself to thermal degradation obviously precedes the possibility for the hologram to exhibit at least the same level of thermal stability. Such precondition for achieving long term thermal stability of the photochromic holograms had been fulfilled with the appearance of diarylethenes compounds [5], which offer advantages of low thermal degradation, high fatigue resistance and multiple write-erase-rewrite cycles. Recently an efficient volume hologram recording in the diarylethene doped bulk polymers had been reported [6, 7] and such hologram thermal degradation properties [6, 8] were studied. It was found that at moderately elevated temperatures, such up to about 60 C hologram's diffraction efficiency does not degrade -even though some initial drop in the diffraction efficiency is possible, but then hologram stabilizes at the level of about 0.6 of its initial value. There was no hologram degradation detected when operating at room temperature. Such appreciable performance itself of the host-guest system deserved exploration of possible practical applications particularly combined with the high photo-fatigue resistance and associated with that multiple rewriting cycles of the holograms. However more demanding applications, which require thermal stability at temperatures up to 100 C were still not accommodated in Refs.[6-8]
due to identified diffusion of the dye between holographic planes and caused by this diffusion hologram degradation/erasure at temperatures above 60 C. One possible route Z

to improve thermal stability of the hologram was suggested in Ref.[6, 8] as covalent bonding of the photochromic dye to the polymer matrix. This concept was the direct logical consequence of the suggested interpretation of the hologram erasure due to enhanced by the temperature elevation diffusion of the dye. Still such concept remained unpublished and unverified experimentally. Here the experimental demonstration is presented on the enhancement of the thermal stability of the photochromic holograms via covalent dye bonding to polymer matrix.

2. Chemicals, materials and sample preparation Two types of the materials have been compared: (1) host-guest system of Ref.[8]
and (2) materials with covalent bonding of the dye to the polymer matrix described herewith. The dyes for both systems, namely KMV-129 (without acrylic bond to form host-guest system of Ref.[8]) and KMV-129A (the one with acrylic bond considered herewith), were supplied by Prof. Robert P. Lemieux of Queen's University.

F F
F F
F F
S S
CaH9O OC<HeOOCCH=CH2 W llvise F F
F F
F F
I ~ S _S
C4H9O OCaHaOOCCH=CH2 Fig.1. Chemical structure, photochromic transformation of diarylethene dye KMV-129A of chemical structure acryl-1,2- bis- [5'- (4"- butyl- oxy-phenyl)- 2'-methyl -thien -3'-yl) perfluoro-cyclo-pentene. An initial state A transforms into B
via UV
light absorption, while reverse transition occurs via absorption of visible light.

A dye acryl-1,2- Bis- [5'-(4"-butyloxyphenyl)-2'-methylthien-3'-yl) per-fluoro-cyclo-pentene was synthesized with the chemical structure shown in Fig, 1. In both cases, i.e. for the dye with [8] and/or without acrylate group attachment, the ring-opened form A
of the dye absorbs only in the UV region of the spectrum due to reduced conjugation of the ic-system. Irradiation with UV light results in photocyclization to the ring-closed form B, which absorbs (see Fig.2) in the visible region of the spectrum (450-750 nm) due to increased conjugation of the n-system. Irradiation with UV light results in photocyclization to the ring-closed form B, which absorbs in the visible region of the spectrum (450-750 nm) due to increased conjugation of the n-system.

0.8 V ------ B 1 0.6 ------ ------L
0.4 ---------0.2 --------- p ~~
O

wavelength, nm Fig.2. Absorption spectra of the photochromic dye KMV-129A attached to the polymer matrix (concentration 0.15 M; sample thickness ca. 30 m). The initial state A is transformed into activated state B via UV light absorption, while de-activation occurs via exposure to visible light, as in Fig. 1.

The same procedure was followed to prepare both types of the recording materials (1) host-guest systems, were photochromic dye is physically dispersed in polymer matrix, and (2) systems in which the dye is covalently attached to the polymer matrix.
However, availability of the acrylate group in the dye molecule KMV-129A made it possible for the dye to participate in free-radical initiated co-polymerization reaction with the monomer, Y

forming polymer chains with statistically distributed dye as a main-chain links inside and which can be presented schematically as:

where M is the monomer and D(a) is the photochromic dye with the acrylate group.
In both cases to prepare the photochromic polymer system, the dye was initially dissolved in the diacrylate monomer SR9008 (Sartomer, Co) by stirring the mixture for 1 hour.
Homogeneity of the solution was visually verified. Then a thermal initiator (azo-bis-azobutyronitrile, AIBN) was added and mixed in by stirring. The solution was then passed through a microporous filter to eliminate the dust and non-dissolved inclusions, if any. Thus prepared, the solution was placed in a glass cuvette and a thermal polymerization process was conducted at 70 C for several hours to convert the monomer into the polymer via free radical polymerization process. The thickness of the cuvette determined the sample thickness d. After completion of the curing, the sample was removed from the cuvette by disassembling its walls, thus resulting in a free-standing sample. Activation of the photochromic dye was carried out by illumination of the sample with UV light at 365 nm using a mercury lamp, which produced a visible absorption band in the material due to the photoinduced transformation A + hv -> B of the dye molecule.

3. Holographic recording and testing procedures Holographic recording has been performed by exposing the polymeric sample in the two-beam set-up (same as in Ref.[4] shown there in Fig.3) with Ar3+ ion laser as a radiation source at 514.5 nm. Probing of the hologram recording was performed in-situ at 830 nm by a laser diode aligned at Bragg angle condition. The exposure time was regulated by a beam shutter. Rotating stage allowed angular selectivity measurement after recording. The temperature stability of the recorded grating and that of the photochromic material was tested by heating up the hologram and control sample (pre-activated by UV irradiation). Measurements were carried out of changes in the ~
S

absorption of the sensitive material (control sample), as well as diffraction efficiency (DE) and angular selectivity of the hologram after storing a hologram at elevated temperature in the oven for selected periods of time. A comparison has been conducted between the absorption decay of the control sample and diffraction efficiency of the hologram to establish the relationship between hologram thermal stability and that of the dye in polymer matrix (either attached or not).

3. Results and discussion The photochromism of both diarylethene derivatives used in this work is due to the photoinduced transformation of the molecule from state A to state B, which differs in terms of their absorption spectra (see Fig.1). Activation of the prepared dye doped and dye-attached polymer materials by UV light (365 nm) caused the A -> B
transition, with the sample acquiring a bluish color and absorption in the visible over the range 450-700 nm, with the maximum absorption at ca. 590 - 600 nm.

Exposure of the UV-activated samples to the interference pattern in the two-beams set up [8] caused a reverse B -> A photochromic transition and a respective shift of the absorption band to occur in the exposed areas. According to the Kramers-Kronig relation [10, 11] this shift causes a change of the refractive index in the exposed areas, i.e.
a drop in the value of the refractive index for visible and near IR spectrum.
Therefore, the recorded holograms have two contributions - from amplitude and from phase counterparts. Within the absorption band, an amplitude component of the hologram reaches its maximum; outside the absorption band, particularly in the near IR
region the amplitude hologram vanishes but the phase hologram dominates. For most applications, the amplitude counterpart plays a negative role due to its contribution to accompanied absorption losses. Contrary to that, the phase hologram contribution is most attractive due to its potential for efficient phase shift of the incident optical waves and resulting higher diffraction efficiency.

In-situ formation of the holograms was monitored [8] by a diode laser readout beam at 830 nm, which is outside the absorption band and therefore does not affect the recording process. A 1:1 beam ratio was used in the recording experiment with equal optical path differences between the beams, which produced a high contrast interference pattern (nearly 1).
Recorded were holograms with spatial frequency of 20001ines/mm.
At a beam diameter D=0.5 cm and output power of recording laser Po,,,=43 mW, the exposure intensity was IeJCp= 4P,,õt/7tD2= 219.1 mW/cm2. Under these exposure conditions the DE peak was reached within topt=130 sec of exposure time, which gives an optimal exposure dose of EoPt = l,xp x topt = 28.5 J/cm2, which defines the sensitivity of the recording material.

Parameters of the hologram recording for the host-guest material are described in Ref.[8].
It is seen that the highest efficiency attained was 13.3% at 514.5 nm at concentration level of the dye C=3.77x10-2 M and sample thickness d=140 m. Solubility of the dye with acryl groups attached is found to be much better and allowed concentration level of times larger which indeed resulted in higher diffraction efficiency level of 19% (A=
2000 mm 1 ;k=514.5 nm; d=140 gm ). It is worth while noting that increase of An for these values is lower than 5 time, which means that the parameter of the recording strength An/C for covalently attached dye is lower than that for dispersed dye without covalent bonding to the matrix.

The results of the hologram's thermal stability for the studied photochromic material with the dye attached to the matrix is shown in Fig.3 and can be compared to that of host-guest system in Ref.[8] (see Fig.6 in Ref.[8]). A one-time partial reduction of the hologram's initial diffraction efficiency due to thermal exposure was observed for both -polymers with attached dye as well as host-guest materials. Its distinctive feature is that such reduction takes place after exposure only, when the hologram is subjected to the heat for the first time after recording. Another observed feature of this one-time partial reduction '1 of the DE was its increase with the temperature level. As it is seen in Fig.6 of Ref.[8], the value of this one-time drop in DE is about 20% of the initial level of the DE at 57 C
and it is increasing 2.5 times at 95 C. It is also seen, that such one time reduction of the DE is present to the same extent in both investigated materials - with and/or without dye's attachment. The last feature indicates that dye's covalent bonding to the matrix does not have effect on the observed DE's one-time drop. In both cases, the host guest as well as dye-attached-to-polymer materials the one time DE reduction due to heat adjustment relaxation had comparable value of about 0.5 of hologram's initial DE (see Fig.3 herewith and compare it to Fig.6 of Ref.[8]). It is suggestion of the present article that this process is due to the thermal relaxation of the induced by photochromic transition structural changes in the polymer network surrounding the photochromic dye moieties. In this interpretation such network adjustment act as a hologram fixing procedure, because it results in stabilization of the hologram, so that repeated exposures to the elevated temperature after that would not lead to changes of the diffraction efficiency beyond the natural thermochromism.

w 0.8 ~
0.6 0.4 1 0.2 ~

=

Exposure to elevated temperature, min Fig.3. Effect of the elevated temperatures on photochromic hologram's efficiency for covalently bonded to polymer matrix dye KMV-129A: 1- at 100 C; 2-stability at room temperature. Enhanced long term thermal stability of the recorded holograms is seen.

The distinctive feature between the two types of these recording materials is that a gradual hologram erasure at elevated temperatures was observed for the host-guest system only. This is assumed to be due to the dye's diffusion between highly lit and dark areas. This is confirmed by the fact that the dye integration into polymeric chain, which would naturally prevent the diffusive transport of the dye molecules, have exhibited no thermal degradation within the tested temperature range (which here was up to 100 C).
As it is seen from Fig.3, at temperatures ca. 100 C there was no detectable diffusion type degradation of the hologram after 6 hours of exposure to elevated temperature.
Same heating of the hologram in photochromic host-guest polymer, where dye's diffusion is not counteracted by the covalent bonding, led to a full hologram erasure within 40 minutes.

Thermal fatigue-resistance stays at comparable level for both system up to the temperatures of ca. 60 C. Within this temperature interval host-guest materials offer advantages of flexibility in the experimental testing due to technological adaptability to changes in material. However at temperatures close to 95 C the hologram recording material on photochromic dyes integrated into polymeric chain exhibit outstanding performance in thermal stability of the hologram recording. Such performance is promising practical competitiveness of such materials in devices with stringent requirements for environmental temperature variations. Additional by-product of the high thermal stability of the holographic recording for optical storage application is improved reliability of the material.

4. Conclusions It is the conclusion of this article that a thermal stability of the hologram recorded on photochromic materials is greatly enhanced when photochromic dye become covalently bonded to the polymer matrix. Such demonstration is believed to be made for the first time and has promising practical implications. The potential of photochromic hologram to exhibit high thermal fatigue resistance greatly contributes to its competitiveness in real devices. Two types of the diffraction efficiency's thermal reduction are distinctly demonstrated - a one-time thermal adjustment of the diffraction efficiency as well as a gradual hologram erasure via dye diffusion process. Both types of the diffraction efficiency reduction are present in the host-guest photochromic materials.
Thermal adjustment of the diffraction efficiency is increasing with the temperature reaching value as high as half of the initial value of the hologram's efficiency in both types of the photochromic materials. Polymer network relaxation process is believed to be a major contributor to the process. Covalently bonded photochromic dye based material is exhibiting only one-time efficiency thermal adjustment and no diffusive erasure of the hologram at temperature at least 100 C high.

5. Acknowledgement Chemical synthesis and supplying of photochromic dye KMV-129 with chemical structure 1,2- Bis- [5'-(4"-butyloxyphenyl)-2'-methylthien-3'-yl)-perfluorocyclopentene and a dye KMV-129A with chemical structure acryl-1,2- Bis- [5'-(4"-butyloxyphenyl)-2'-methylthien-3'-yl)perfluorocyclopentene by Kenneth E. Maly, Peng Zhang and Prof.
Robert P. Lemieux of Department of Chemistry, Queen's University, Kingston, Ontario, K7L 3N6, Canada is gratefully acknowledged, which made this work possible.

References 1. D. M. Abakoumov, N. A. Ashurbekov, E. H. Gulanian and A. L. Mikaelian, "Multilayered Holographic Memory," Optical Memory & Neural Networks 15 (4) (1996).

2. A. L. Mikaelian, B. V. Kryzhanovsky, V. K. Salakhutdinov and A. Fonarev, "Holographic Recording in Bacteriorhodopsin by Short Light Pulses,"- Optical Memory & Neural Network 8 (4), 614-622 (1999).
3. B. J. Siwick, O. Kalinina, E. Kumacheva, R. J. D. Miller and J. Noolandi, "Polymeric nanostructured material for high-density three-dimensional optical memory storage,"-Journal of Applied Physics 90 (10), AIP, 5328-5334 (2001) 4. J.-W. Kang, F. Kim and J.-J. Kim, "All-optical switch and modulator using photochromic dye doped polymer waveguides,"- Optical Materials 21 (1-3), 543-548 (2003).
5. M. Irie and K. Uchida, "Synthesis and properties of photochromic diarylethenes with Heterocyclic aryl groups,"- Bull. Chem. Soc. Jpn. 71, 985-996 (1998).

6. Yu. Boiko, "Specialty polymers for optical and photonics technologies,"-Technical Report of March 2003, National Research Council (NRC) of Canada, March 2003 -(submitted to NRC).

7. Yu. Boiko, "Volume hologram recording in diarylethenes," - Proc. SPIE, 2008 (in Press.).

8. Yu. Boiko, "Volume hologram recording in diarylethene doped polymer," -Optical Memory & Neural Networks (2008).

9. K. E. Maly, M. D. Wand and R. P. Lemieux, "Bistable ferroelectric liquid crystal photoswitch triggered by a dithienylethene dopant",- Journal of the American Chemical Society, vol. 124, 7898-7899 (2002) 10. G.W. Burr, "Volumetric Storage" - in: Encyclopedia of Optical Engineering, Ed.:
R.B.Johnson and R.G.Driggers, Marcel Dekker, New York, 2002.

11. R. J.Collier, C. B. Burckhardt, and L. H. Lin, Optical Holography, Academic Press, N.Y., 1971.

Claims

Claim 1.

Material comprising - photochromic dye with at least one unsaturated double bond ...........................................................Ø1-10 mass %
- mixture of crosslinking (met)-acryl ate monomers and/or oligomers with the number of unsaturated double bonds not less then 2 ....................................................... 10 -99 mass %
- diluting (met)-acryl ate monomer with one unsaturated double bond ........................................................... 0 - 70 mass %
- solvent ................................................................ 0 -20 mass %
- Polymeric binder ...................................................... 0 -70 mass %
- Thermal initiator .................................................. 0.001 -10 mass %
- Thermal co-initiator .............................................. 0 - 10 mass %

which allows implementation of the fabrication process including the following stages:
1) dissolving of all solid state components in the solvent and diluting monomer;
2) mixing in homogeneously all the remaining liquid components;
3) filling in the cuvette with the resulting homogeneous solution;
4) carrying on the thermal polymerization process;
5) disassembling the cuvette to produce stand alone sample or leaving the cuvette in-tact to use the produced sample as is (i.e. with the cuvette as part of the device) and which after activation by the UV light allows photochromic recording of holograms stable at temperatures not less than 100 °C.

Claim 2. Material as in the claim 1, where as a photochromic dye the diarylethene is used with the structure where at least one of R1 and R2 or both are -O(CH2) n OOCC(M)=CH2 with n=1, 2, 3, ...and M being -H or -CH3 while the remaining R i (or none) is selected from the series -OH, -OCH3, -OC2H5, -OC4H9, -OC5H11; ... -OC n H2n+1 (i={1,2}; n=0, 1, 2, 3, 4, 5,... ).