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Photochemistry and multiple holographical recording in polymers with photochromic phenoxy-naphthacene-quinone side groups
Y. Photochem, Phoroblol, A: Chem., 76 (1993) 135-141
135
Photochemistry and multiple holographical recording in polymers with photochromic phenoxy-naphthacene-quinone side groups Alexander Zelichenok, Frida Buchholz, Ernst Fischer, Judith Ratner and Valeri Krongauz* Depatimen! of Ogonic Chemby,
The Weizmann Institute of Science, Rehovot 76100 (Israel)
Hans Anneser and Christoph Brguchle Institut fir Physikalische Chemie, Uni**wkYt Miinchen, W-80013Municlt (Germany) (Received March 30, 1993; accepted June 17, 1993)
Abstract The photochromism of phenoxy-naphthacene-quinones in solution was investigated and compared with that of polymers in which one of these compounds was incorporated as a side group, through flexible (CH,)” spacers of various lengths. The polymers studied were polystyrene (PS), poly(methyl methacrylate) (PMMA), and polysiloxane (PSX). Up to 90% photochrome side groups could be incorporated in some of these polymers. The polymers were studied both in solution and as flexible films. The quantum yields in solutions of one of phenoxy-quinones was about 0.6 for truns-~na, and about 0.10 for ana +truns. The photoconversion ~runr+unu is only slightly slowed down in solutions of the polymers, but much more so in films. The rates of photoconversion observed in films, relative to a solution of the photochrome proper were as follows: PS, 12; PMMA, 25; PSX, SO;solution, 100. These rates vary slightly with the concentration of the photochrome but increase with increasing spacer length in PMMA and decrease in PSX. The glass transition temperature may play an important role. MuItiple holographic recording on the polymers was investigated.
1. Introduction The photochromism of certain derivatives of phenoxy-naphthacene-quinone is based on the photo-induced interconversion of the fruns p-quinone form into the anap-quinone form [l] (Scheme 1). The trans form has an absorption peak at around 400 nm, while the ana form is characterized by a double peak in the range 440-480 nm. The photostationary state (PSS) achieved by UV irradiation at up to about 405 nm therefore contains mainly ana, while irradiation above about 460 nm, Photochromic
Transformation
“trans”
of
Nophthaccncquinone
“Sln8”
Scheme 1. ‘Author to whom correspondence should be addressed.
lOlO-6030/93/$6.00
where only ana absorbs, results in complete photoconversion into trans. The absorption spectra of a solution of the pure trans form and the anaenriched form of one derivative are shown later in Fig. l(a). No thermal interconversion frans *(mu takes place at room temperature. In a previous paper [2] we described the synthesis of these compounds and of three polymers in which they were incorporated: polysiloxane (PSX), polystyrene (PS) and poly(methy1 methacrylate) (PMMA). In the preserit paper, we describe the reversible photochemistry of the truns *ana system in solution in deta.il and compare it with those of the photochromic polymers, both in solution and as films. Molar extinction coefficients and approximate quantum yields of the photoconversions were calculated. A possibility of employing the polymers as materials for multiple hoiographic recording was explored. 2. Experimental
details
2.1. Chemicals The syntheses of all compounds and polymers have been described in our earlier paper (21. 0 1993 - Elsevier Sequoia. All rights reserved
136
A. Zelichenok et al. I Photochemisty
and wtltip
The solvents used were toluene (analysis grade), tetrahydrofuran (THF), methyltetrahydro.%ran (MTHF) (for work at low temperatures) and methyl reti-butylether, which has the advantage of not forming peroxides.
fc holographical recording in polymcts
’ ~crivntlvcr al
6.Phenoxy.5,12.Nnph~haccnequlnanc
1.R=
2.2. Photochemistry and spectroscopy Absorption spectra were measured on a Cary 2200 spectrophotometer. For flash photolysis, a more or less conventional microsecond home-built set-up was used. Both instruments were equipped with cryostats, allowing measurements down to -160 “C 131. Irradiation could be carried out either in situ in the sample compartment of the spectrophotometer or outside it. Light sources were either a 100 W medium pressute mercury lamp (Mazda) or a 100 W tungsten--iodine lamp, both being used in Wild illuminators. Appropriate light filters were used to isolate the mercury lines at 313, 366 and 405 nm. An interference filter with a transmission peak at 474 nm was used with the tungsten lamp to achieve complete ana-+trans conversion. For flash photolysis experiments, cells of 20 nm light path (measuring beam) were used, made of rectangular glass or Suprasil tubing (crosssection, 4 rnrn~ 16 mm) with fused-on windows 141. ’
-Nl$
2. R=
-CH&COH
A
Scheme 2.
Pofymcrs Conlainfng
Pholochrome
B(4)
2.3. Actinometty Aberchrome 540 was used at 313 and 365 nm as described previously [S, 61. The reverse reaction was employed to estimate the light intensity at 477 nm [6]. 2.4. Other methods The glass transition temperatures Tg, of the polymers were measured by differential scanning calorimetry using a Mettler TA-3000 instrument. Efforts to determine the molecular weight of the polymers by high performance liquid chromatography did not give meaningful results.
IV
Scheme 3.
3. Results 3.1. Photochemistry and spcctya of the photochromic compounds Type A derivatives (Scheme 2) were only slightly photochromic, but all type B derivatives showed normal photochromism. However, only compound B-4 in Scheme 2 could be incorporated in the polymer. Since this compound contains a free NH2 group, its protected derivative C was used for comparison with the photochromic polymers depicted in Scheme 3. Figure l(a) shows the photo-
induced spectral changes in an ether soiution (methyl teti-butylether) of C. Isosbestic points were observed at 392, 360, 336, 270 and 252 nm; the longest absorption peak of the trans form is at 391 nm, while the ana form is characterized by a double peak at 467 and 440 nm. The peaks and isosbestic points in THF and in toluene are shifted S-15 nm to longer wavelengths. Solutions flushed by either oxygen or argon gave similar results, both spectrally and kinetically. Experiments with a solution in MTHF showed that photointerconversion takes place at -100 “C at similar rates
A. Zelielrenok et al. I PItorochetnistty and multiple Iiolrgmphica? recording in po!ytnets I
LVcwelengihhrn
(4 1.4 F--*---
@I
L
,
t
1
I
I
Wavelengthhm
Fig. 1. (a) Spectral changes in an ether solution of compound C. irradiated at 365 nm: curve starting just below 0.4 absorbance at 250 nm wavelength was obtained before irradiation; other curves were recorded after 1 min irradiation each time. (b) PSS achieved at 313 nm: curve 1, spectrum of the rr0rr.rform; curve 2, spectrum following irradiation “to completion” (PSS); curve 3, extrapolated curve for pure am, assuming 85% conversion into ano in curve 2.
as at ambient temperature. No evidence was found for the existence of a long-Iived transient in the photoconversion truns 3 ana by static methods, even at - 100 “C. In order to estimate the extent of photoconversion in the PSS established by irradiation at a certain wavelength, and thereby the absorption spectrum of the pure ana form, we used the following considerations, as described in
137
an earlier publication 171. During the conversion fvans dana, the absorbance D decreases in the spectral region around 300 nm. In Fig. l(b), depicting the PSS achieved by 313 nm irradiation, D3”’ decreases by about 70% of its initial value (i.e. in the pure trans form). Obviously D300 of ana cannot be below zero, and therefore the actual spectrum of pure ana must be somewhere between the experimental curve 2 at PSS and a curve obtained by extrapolation, assuming that D300= 0, i.e. a decrease of 100%. In the absence of additional information, we may assume that the true value of 03Mo”(I is midway between these two extremes, with I1300aaa= 15% of its value for the trans form. This amounts to assuming that the PSS (curve 2 in Fig. l(b)) contains 85% ana and 15% trans. Curve 3 in this figure was obtained by extrapolation, employing this assumption. Irradiation at A > 430 nm causes complete conversion into the trans form, which does not absorb at these wavelengths. The molar extinction coefficient 4, of pure truns can thus be determined by measuring the absorption spectrum of a solution of a known molar concentration, after irradiation at A>430 nm. The ratio of the peak of curve; 1 to that of curve 3 in Fig. l(b) then allows calculation of the desired EP”ll values. The following E vaiues were thus obtained for a solution in toiuene: Pg8,mlts= 1.85 X l@ mol-’ dm3 cm-’ and ~~~,,=5.4X lo3 mol-’ dm3 - ’ assuming that the relative E values at the Elks are the same in both toluene and ether. These Evalues are about a third of those reported by Gerasimenko et al. [8], who used the selective irreversible reaction of amines with phenoxy-anaquinone leading to a stable product, amino-anaquinone, while the &am form does not react markedly with amines. We also employed this method to check our above E values for the ana form; decylamine was added to a solution of C, after PSS had been achieved by 405 nm irradiation. Absorption of the new band at A,,=536 nm corresponds to a concentration of amino-ana-quinone equal to the steady state concentration of the ana form of C (Fig. 2). Further irradiation of the solution in the presence of the amine resulted in additional growth of Dsz reaching a limit value (Fig. 2, curve 4). Since amino-ana-quinone is a stable product, its concentration should be equal to the starting concentration of C, allowing an estimation of Z36 of the product. From this we estimated the PSS concentration of the product and hence of the ma form (83%), leading to p70 dm3 cm-‘, which almost ana =5.1 X lo3 mol-’ coincides with our above calculation, 5.4X 103 mol - I dm3 cm- I. We fail to understand the reason
138
A. Zclichenok et al. I Photochemistry and multiple IrolograP?~icnlrecording in polymers
Wavelengt h/nm Fig. 2. Reaction of the ana form of compound C with decylamine: curve 1, spectrum of the pure frarrs form; curve 2, foilowing 405 nm irradiation “to completion”; curve 3, same after addition of decylamine, which reacts with the atm form to yield decylaminophenoxy naphthacene-quinonc; curve 4, solution of curve 3 following further irradiation “to compIetion” at 405 nm, converting the remaining lrarls in solution 2 into ano, which reacts with the excess dccylaminc.
for the huge discrepancy between thgse of Gerasimenko et al.
our results and
3.2. Quantum yields
TI’heyields for tram +ana at 313 and 365 nm irradiation were calculated, using light intensity values determined by Aberchrome 540 actinometry [5, 61. The experimental rates of photoconversion and the E values of the ana form have been described in the previous paragraph. The light intensity at 405 nm, relative to that at 365 nm, was estimated with a Rhodamine quar,tlun converter. The resulting quantum yields for truns 4unu were almost the same at 313, 365 and 405 nm, namely 0.60 f 0.10. The quantum yield for ana + truns was measured by irradiation at 477 nm, compared with the reverse reaction of Aberchrome 540 irradiated at the same wavelength. Taking the yield of Aberchrome at 477 nm as 0.0635 (63, the yield for ana +trans is 1.56 times greater, i.e. 0.10. Presumably the yield at shorter wavelengths is similar, as shown by the high extent of photoconversion at the PSSs attained by irradiation at 405 and 365 nm, c!ose to the isosbestic points.
3.3. Flash photo&k In an attempt to detect possible intermediates Of transients during the photoconversion truns 4 ana, flash photolysis experiments were carried out with solutions in MTHF, both at ambient temperature and at -100 “C. The light emitted by the air-filled discharge lamps was filtered by Corning 7-60 filters having a transmission peak at 360 nm. The measuring light beam was filtered to transmit above 440 run only, to prevent formation of unu by the monitoring light. After every two to three flashes, the solution was irradiated with visible light to reconvert the ana formed during the flashes into trans. As in the static experiments, solutions flushed with oxygen and with argon yielded the same quantity of ana per flash. At each wavelength of the monitoring light, a kinetic curve was taken at a short and a long time scale, as illustrated in Fig. 3. In all cases, we obtained only the.“step-type” curves shown, indicating the “immediate” formation of ana on the time scale of our set-up, which could detect transients of above 15 ps half-life. The single exception was a short-lived transient detected at 630 nm (where una does not absorb) but only at -100 “C (c$ Fig. 3(D)). Its intensity was about 10% of that at the peak, but no parallel “growing-in” of the absorption at the peak of the ana form was observed; thus there may be no connection between this signal and the formation of ana. 3.4. Photochromic polymers Their synthesis has been reported [2]. Scheme 3 describes the three types of polymer investigated:
6-
54
p :: P g
2
d O-
Fig. 3. The spectrum of compound C measured by flash photolysis at room temperature (main curve). The insets are kinetic curves taken at -100 “C (A and B at 480 nm and C and D at fX0 nm). At both wavelengths, curves were taken on both short and long time scales, as shown.
A. Zelichenok et al. I Photochemistryand rnrtlfble holographicalrecording in ,vo!ymers
PSX, PS and PMMA. For each type of polymer, we studied the products of incorporation of the photochromic compound B-4 (Scheme 2) through side chains of various lengths (except for PS) and at various concentrations. The polymers were studied both as solutions in THF or toluene, and in the form of films formed by dip coating of glass slides. In each case the rates of the photoconversion fru)t~--,ana were measured, taking into account the absorption at the wavelength used for irradiation, 365 or 405 nm. The absorption spectra were, in all cases, similar to those of solutions of the photochrome. In all films of the photochromic polymers, the reverse reaction ana * trans also takes place, as in solutions of the photochrome proper. The clearest results regarding the kinetics of the fauns *unu phototransformation were obtained for PSX, while for PMMA and PS a substantial scattering of the initial rates of the conversion was observed. Apparently, this stems from a very significant decrease in the degree of polymerization of PMMA and PS with increasing concentration of the active ester monomer in the polymerization mixture [2], since succinimide active ester should be a strong inhibitor of free-radical polymerization. Note that only PSX was obtained by a polymer analogous reaction from a PSX with a given degree of polymerization, 35. In a typical experiment for PSX, a solution of compound C (Scheme 3) in ether, a solution of polymer I, n =8 (Scheme 3), in ether, and a film of this polymer were irradiated at 365 nm under identical conditions inside the sample compartment of the spectrophotometer. The relative initial rates of anu formation were as follows: solution of compound C, 100; solution of polymer, 66; film, 44. Note that the colour decay did not obey firstorder kinetics in the films and the initial rates were calculated from the initial slope of the colourfading curve. The relative rates measured in toluene solutions of compound I, n =8 (Scheme 3), at various con. * ccat:at;ons of the photochrome :n the po!ymer PSX compared with a solution of C, are given in Table 1 (365 nm irradiation). TABLE 1. Relative rates at various photochrome in PSX Photochrome
Molar concentration Relative initial rate
concentrations
Photochrome C 100 (i.e. pure) (%)
10 60
20 60
40 5s
50 47
100
TABLE
2. Effect of n and 7’s on relative rate
n in compound 1 4 8
139
I
Relative rate 100 74 61
initial &) 2 8 38
The effect of the length of the spacer chain in films, as expressed by n in formula I (Scheme 3), is shown in Table 2, in which we also report the corresponding glass transition temperature T,. The concentration was about 30%. Again the effect was not large. T. of the polymer substituted only by C7H15 groups (instead of the photochromic group) was -90 “C. Tg at photochrome concentrations above 10% is almost independent of the concentration, with values of 30-40 “C. The relative rates of frans +una photoconversion at 405 nm of films of the three photochromic polymers I (n = 4), II and III (k = 5)‘and a solution of C in ethzr were a.s follows: PS, 12; PMMA, 25; PSX, 50; solution, 100. The molar concentration of the photochrome was about 12%. Statistical data handling for all polymers with different spacers and concentrations of the photochrome gives a very similar trend for the initial rate: PSX, 7.3 f 1.3; PMMA, 3.3i0.5; PS, 2.5ItO.6. At the highest molar concentration of photochrome in PMMA and in PS, the two polymers are obviously almost identical, because most of the side groups are replaced by the photochrome. 3.5. Holographic recording with the photochromic polymers The possibility has been mentioned of employing 6-phenoxy-5,lZnaphthacene-quinone dissolved in PMMA as a material for holographic recording [l]. In order to examine the feasibility of such an application, we performed the holographic recording on films of PMMA (20% of the photochrome) and PSX (30% of the photochrome). Holographic gratings were written with wavelengths of 413 nm (for tran.r +anu conversionj and 482 nm (for ana --) trans conversion) of Kr + laser and non-destructive read-out with a He-Ne laser at 632.8 nm (for details of the holograph technique, see refs. 9 and 10 and references cited therein). Maximum diffraction efficiencies of 0.2% and 0.5%, were obtained for rrun.s +ana and anu + fruns conversions respectively for both polymers. Kinetics of the hologram growth for PSX and PMMA (writing with 413 nm) are shown in Fig. 4. A rapid
A. Zelichcnok ef al. I Photochemisty
140
0.06 3 e
ii? 0.06
0.6 t
cB 0.04 5 0.03 E “y 0.02 0.01 0.00
’ 0
I
20
40 TIME
BQ IsI
80
100
Fig, 4. Hologram formation at 413 nm (writing beam intensity, 2.0 W cm-l): cutve 1, PSX (30% photochrome); curve 2, PMMA (20% photochrome).
10
0
20
30
40
50
e’o
NCNBER OF CYCLES
Fig, 6. Dependence cording wavelength, PMMA.
of the hologram formation etticiency (re482 nm) on the number of cycles for 20%
0.40
‘;; &
0.30
z E 0.20 E 0
20
40 TIME
80 IsI
80
Fig. 5. Write-erase cycles on 20% PMMA, corresponding to cycles 40-45 in Fig. 6 (au., arbitrary units): curve 1, recording on clna form with 482 nm, intensity of the writing beams of 0.14 W cm-*; curve 2, switching light off; curve 3, erasing with UV tight.
drop to the somewhat lower efficiency could be observed after switching off the writing beam. Holograms recorded on PMMA could be completely erased by irradiation with suitable light, Le. for recording by frans form (413 nm), erasing with visible light, and for recording by ana form (482 nm), erasing with UV light. A number of write (482 nm) and erase (UV light) are shown for PMMA. films in Pig. 5. By contrast, the holograms on PSX films cannot be erased completely by subsequent illumination. Figure 6 shows that the efficiency of holograms recorded on the same spot on a sample of PMMA declined after a number of cycles even though complete erasure by illumination is accomplished each time. The intensity dependence of the hologram formation shown in Fig. 7 indicates that increasing intensity accelerates both the efficiency growth and the photoinduced fatigue of the hologram. The hologram can be fixed by keeping it in NH, vapour over ammonia solution, which converts the
%
0.10
0.00
0
5
10
15
20
TIME IsI Fig. 7. Dependence of the hotogram formation efficiency on intensities of the recording beams (20% PMMA; recording wavelength, 482 nm): curve 1, 14 mW cm-‘; curve 2, 22 mW cm-‘; curve 3, 34 mW cm-$ curve 4, 88 mW cm-‘; curve 5, 140 mW cm-l; curve 6, 280 mW cma2; curve 7, 560 mW cme2.
area form into amino-una-quinone. This procedure increased the diffraction efficiency three times, to about 1.5%. 4. Discussion The main result of the present investigation is that the polymers of all three types in which the phenoxy-naphthacene-quinone was incorporated exhibit qualitatively the same photochromism as the quinone, both in solution and as films. The quantum yield in the films is between 10 and 50% of that in sohrtions of the quinone, probably as a result of the high viscosity of these media. Indeed, the variation in the glass transition temperature T, seems to reflect, to some extent,
A. Zelichcnok et al. i Photochemistry and muIt@le hologruphical wcording in polymers
the variation in the viscosity of the films. The rate of photoconversion decreases with increasing T,, i.e. increasing viscosity, irrespective of the cause of sech an increase (photochrome concentration, length of the (CH& chain, or type of polymer). In solutions of the polymers, the viscosity factor obviously no longer applies. In dilute solutions, the main factor responsible for differences between such solutions and solutions of the quinone proper is the extent to which the photoconversion of the photochromic side group is hindered by the surrounding polymer. From the results obtained, we conclude that this effect amounts to no more than 50% and decreases with increasing length of the chain by which the photochrome is connected to the polymer backbcne. Only at the highest concentration of photochrome does some effect of the interaction between neighbouring photochromic side groups seem to appear in some cases. For the purpose of applications, it is important to note that films with a very high content of photochrome can be prepared, while still retaining most of the mechanical properties of the polymers. Moreover, the insignificant irreversible photochemical side reactions and the absence of thermal interconversion frans +una are of obvious practical advantage. The holographic experiments demonstrated the possibility of erasable recording, in principle, although the low efficiency and insufficient fatigue resistance of the materials represent substantial problems for the application. Finally, reasonable estimates of the absorption sp.Tctrum of the pure ana form and of the quantum yields in both directions for compound C were obtained: about 0.6 and 0.1 for truns dana and ana + tram respscrivtily.
141
Regarding the mechanism of the photoconversion, the fact that rates of photoconversion are not affected by oxygen indicates that a triplet transient is not involved. The flash photolysis results show that, even at -100 “C, the photoformation of the mu modification in solution is instantaneous on the time scale of the equipment used, i.e. about 15 p. Of course, this does not rule out the existence of shorter-lived transients between singlet-excited truns and ground-state ana. Acknowledgment Support from the German-Israeli Foundation for Scientific Research and Development is gratefully acknowledged. References A.V. El’tsov (ed.), OtgunicPhotochromes, Consultants Bureau, New York. 1990, p_ 210. F. Buchholtz, A. Zeiichenok and V. Krongauz, Macromolecules,
26 (1993) 906. E. Fischer, Mol. Photochem., 2 (1970) 99. E. Fischer, Mol. Photochenz., 6 (1974) 1. H.G. Helter and I.R. Langan. EPA NW.& (October 1981) 71. IUPACCommission on Photochemistry, ChemicalAcfinometry, 1988, in Pure Appl Chem., 61 (1989) 187. K. Macda and E. Fischer, Helv. Chim. Actu, 66 (1983) 1961, and references cited therein. Yu.E. Gerasimenko, A-A. Parshutkin, N.T. Poteleschenko, V.P. Poteleschenko and V.V. Romanov, Zh. Priki. Spectrosk, 30 (1979) 954. 9 J. Pinsl, M. Gehrtz and C. Brluch1e.J. Phys. Chem., 90 (1986)
6754. 10 C. Briiuchle and H. Anneszr, Holographic spectroscopy and holographic information recording in polymer matrices, in J.P. Fouassier and J.F. Rabek (eds.), Losers in Polymer Science and Tcchnolow: AppIicafions, CRC Press, Boca Raton, FL, 1990.
135
Photochemistry and multiple holographical recording in polymers with photochromic phenoxy-naphthacene-quinone side groups Alexander Zelichenok, Frida Buchholz, Ernst Fischer, Judith Ratner and Valeri Krongauz* Depatimen! of Ogonic Chemby,
The Weizmann Institute of Science, Rehovot 76100 (Israel)
Hans Anneser and Christoph Brguchle Institut fir Physikalische Chemie, Uni**wkYt Miinchen, W-80013Municlt (Germany) (Received March 30, 1993; accepted June 17, 1993)
Abstract The photochromism of phenoxy-naphthacene-quinones in solution was investigated and compared with that of polymers in which one of these compounds was incorporated as a side group, through flexible (CH,)” spacers of various lengths. The polymers studied were polystyrene (PS), poly(methyl methacrylate) (PMMA), and polysiloxane (PSX). Up to 90% photochrome side groups could be incorporated in some of these polymers. The polymers were studied both in solution and as flexible films. The quantum yields in solutions of one of phenoxy-quinones was about 0.6 for truns-~na, and about 0.10 for ana +truns. The photoconversion ~runr+unu is only slightly slowed down in solutions of the polymers, but much more so in films. The rates of photoconversion observed in films, relative to a solution of the photochrome proper were as follows: PS, 12; PMMA, 25; PSX, SO;solution, 100. These rates vary slightly with the concentration of the photochrome but increase with increasing spacer length in PMMA and decrease in PSX. The glass transition temperature may play an important role. MuItiple holographic recording on the polymers was investigated.
1. Introduction The photochromism of certain derivatives of phenoxy-naphthacene-quinone is based on the photo-induced interconversion of the fruns p-quinone form into the anap-quinone form [l] (Scheme 1). The trans form has an absorption peak at around 400 nm, while the ana form is characterized by a double peak in the range 440-480 nm. The photostationary state (PSS) achieved by UV irradiation at up to about 405 nm therefore contains mainly ana, while irradiation above about 460 nm, Photochromic
Transformation
“trans”
of
Nophthaccncquinone
“Sln8”
Scheme 1. ‘Author to whom correspondence should be addressed.
lOlO-6030/93/$6.00
where only ana absorbs, results in complete photoconversion into trans. The absorption spectra of a solution of the pure trans form and the anaenriched form of one derivative are shown later in Fig. l(a). No thermal interconversion frans *(mu takes place at room temperature. In a previous paper [2] we described the synthesis of these compounds and of three polymers in which they were incorporated: polysiloxane (PSX), polystyrene (PS) and poly(methy1 methacrylate) (PMMA). In the preserit paper, we describe the reversible photochemistry of the truns *ana system in solution in deta.il and compare it with those of the photochromic polymers, both in solution and as films. Molar extinction coefficients and approximate quantum yields of the photoconversions were calculated. A possibility of employing the polymers as materials for multiple hoiographic recording was explored. 2. Experimental
details
2.1. Chemicals The syntheses of all compounds and polymers have been described in our earlier paper (21. 0 1993 - Elsevier Sequoia. All rights reserved
136
A. Zelichenok et al. I Photochemisty
and wtltip
The solvents used were toluene (analysis grade), tetrahydrofuran (THF), methyltetrahydro.%ran (MTHF) (for work at low temperatures) and methyl reti-butylether, which has the advantage of not forming peroxides.
fc holographical recording in polymcts
’ ~crivntlvcr al
6.Phenoxy.5,12.Nnph~haccnequlnanc
1.R=
2.2. Photochemistry and spectroscopy Absorption spectra were measured on a Cary 2200 spectrophotometer. For flash photolysis, a more or less conventional microsecond home-built set-up was used. Both instruments were equipped with cryostats, allowing measurements down to -160 “C 131. Irradiation could be carried out either in situ in the sample compartment of the spectrophotometer or outside it. Light sources were either a 100 W medium pressute mercury lamp (Mazda) or a 100 W tungsten--iodine lamp, both being used in Wild illuminators. Appropriate light filters were used to isolate the mercury lines at 313, 366 and 405 nm. An interference filter with a transmission peak at 474 nm was used with the tungsten lamp to achieve complete ana-+trans conversion. For flash photolysis experiments, cells of 20 nm light path (measuring beam) were used, made of rectangular glass or Suprasil tubing (crosssection, 4 rnrn~ 16 mm) with fused-on windows 141. ’
-Nl$
2. R=
-CH&COH
A
Scheme 2.
Pofymcrs Conlainfng
Pholochrome
B(4)
2.3. Actinometty Aberchrome 540 was used at 313 and 365 nm as described previously [S, 61. The reverse reaction was employed to estimate the light intensity at 477 nm [6]. 2.4. Other methods The glass transition temperatures Tg, of the polymers were measured by differential scanning calorimetry using a Mettler TA-3000 instrument. Efforts to determine the molecular weight of the polymers by high performance liquid chromatography did not give meaningful results.
IV
Scheme 3.
3. Results 3.1. Photochemistry and spcctya of the photochromic compounds Type A derivatives (Scheme 2) were only slightly photochromic, but all type B derivatives showed normal photochromism. However, only compound B-4 in Scheme 2 could be incorporated in the polymer. Since this compound contains a free NH2 group, its protected derivative C was used for comparison with the photochromic polymers depicted in Scheme 3. Figure l(a) shows the photo-
induced spectral changes in an ether soiution (methyl teti-butylether) of C. Isosbestic points were observed at 392, 360, 336, 270 and 252 nm; the longest absorption peak of the trans form is at 391 nm, while the ana form is characterized by a double peak at 467 and 440 nm. The peaks and isosbestic points in THF and in toluene are shifted S-15 nm to longer wavelengths. Solutions flushed by either oxygen or argon gave similar results, both spectrally and kinetically. Experiments with a solution in MTHF showed that photointerconversion takes place at -100 “C at similar rates
A. Zelielrenok et al. I PItorochetnistty and multiple Iiolrgmphica? recording in po!ytnets I
LVcwelengihhrn
(4 1.4 F--*---
@I
L
,
t
1
I
I
Wavelengthhm
Fig. 1. (a) Spectral changes in an ether solution of compound C. irradiated at 365 nm: curve starting just below 0.4 absorbance at 250 nm wavelength was obtained before irradiation; other curves were recorded after 1 min irradiation each time. (b) PSS achieved at 313 nm: curve 1, spectrum of the rr0rr.rform; curve 2, spectrum following irradiation “to completion” (PSS); curve 3, extrapolated curve for pure am, assuming 85% conversion into ano in curve 2.
as at ambient temperature. No evidence was found for the existence of a long-Iived transient in the photoconversion truns 3 ana by static methods, even at - 100 “C. In order to estimate the extent of photoconversion in the PSS established by irradiation at a certain wavelength, and thereby the absorption spectrum of the pure ana form, we used the following considerations, as described in
137
an earlier publication 171. During the conversion fvans dana, the absorbance D decreases in the spectral region around 300 nm. In Fig. l(b), depicting the PSS achieved by 313 nm irradiation, D3”’ decreases by about 70% of its initial value (i.e. in the pure trans form). Obviously D300 of ana cannot be below zero, and therefore the actual spectrum of pure ana must be somewhere between the experimental curve 2 at PSS and a curve obtained by extrapolation, assuming that D300= 0, i.e. a decrease of 100%. In the absence of additional information, we may assume that the true value of 03Mo”(I is midway between these two extremes, with I1300aaa= 15% of its value for the trans form. This amounts to assuming that the PSS (curve 2 in Fig. l(b)) contains 85% ana and 15% trans. Curve 3 in this figure was obtained by extrapolation, employing this assumption. Irradiation at A > 430 nm causes complete conversion into the trans form, which does not absorb at these wavelengths. The molar extinction coefficient 4, of pure truns can thus be determined by measuring the absorption spectrum of a solution of a known molar concentration, after irradiation at A>430 nm. The ratio of the peak of curve; 1 to that of curve 3 in Fig. l(b) then allows calculation of the desired EP”ll values. The following E vaiues were thus obtained for a solution in toiuene: Pg8,mlts= 1.85 X l@ mol-’ dm3 cm-’ and ~~~,,=5.4X lo3 mol-’ dm3 - ’ assuming that the relative E values at the Elks are the same in both toluene and ether. These Evalues are about a third of those reported by Gerasimenko et al. [8], who used the selective irreversible reaction of amines with phenoxy-anaquinone leading to a stable product, amino-anaquinone, while the &am form does not react markedly with amines. We also employed this method to check our above E values for the ana form; decylamine was added to a solution of C, after PSS had been achieved by 405 nm irradiation. Absorption of the new band at A,,=536 nm corresponds to a concentration of amino-ana-quinone equal to the steady state concentration of the ana form of C (Fig. 2). Further irradiation of the solution in the presence of the amine resulted in additional growth of Dsz reaching a limit value (Fig. 2, curve 4). Since amino-ana-quinone is a stable product, its concentration should be equal to the starting concentration of C, allowing an estimation of Z36 of the product. From this we estimated the PSS concentration of the product and hence of the ma form (83%), leading to p70 dm3 cm-‘, which almost ana =5.1 X lo3 mol-’ coincides with our above calculation, 5.4X 103 mol - I dm3 cm- I. We fail to understand the reason
138
A. Zclichenok et al. I Photochemistry and multiple IrolograP?~icnlrecording in polymers
Wavelengt h/nm Fig. 2. Reaction of the ana form of compound C with decylamine: curve 1, spectrum of the pure frarrs form; curve 2, foilowing 405 nm irradiation “to completion”; curve 3, same after addition of decylamine, which reacts with the atm form to yield decylaminophenoxy naphthacene-quinonc; curve 4, solution of curve 3 following further irradiation “to compIetion” at 405 nm, converting the remaining lrarls in solution 2 into ano, which reacts with the excess dccylaminc.
for the huge discrepancy between thgse of Gerasimenko et al.
our results and
3.2. Quantum yields
TI’heyields for tram +ana at 313 and 365 nm irradiation were calculated, using light intensity values determined by Aberchrome 540 actinometry [5, 61. The experimental rates of photoconversion and the E values of the ana form have been described in the previous paragraph. The light intensity at 405 nm, relative to that at 365 nm, was estimated with a Rhodamine quar,tlun converter. The resulting quantum yields for truns 4unu were almost the same at 313, 365 and 405 nm, namely 0.60 f 0.10. The quantum yield for ana + truns was measured by irradiation at 477 nm, compared with the reverse reaction of Aberchrome 540 irradiated at the same wavelength. Taking the yield of Aberchrome at 477 nm as 0.0635 (63, the yield for ana +trans is 1.56 times greater, i.e. 0.10. Presumably the yield at shorter wavelengths is similar, as shown by the high extent of photoconversion at the PSSs attained by irradiation at 405 and 365 nm, c!ose to the isosbestic points.
3.3. Flash photo&k In an attempt to detect possible intermediates Of transients during the photoconversion truns 4 ana, flash photolysis experiments were carried out with solutions in MTHF, both at ambient temperature and at -100 “C. The light emitted by the air-filled discharge lamps was filtered by Corning 7-60 filters having a transmission peak at 360 nm. The measuring light beam was filtered to transmit above 440 run only, to prevent formation of unu by the monitoring light. After every two to three flashes, the solution was irradiated with visible light to reconvert the ana formed during the flashes into trans. As in the static experiments, solutions flushed with oxygen and with argon yielded the same quantity of ana per flash. At each wavelength of the monitoring light, a kinetic curve was taken at a short and a long time scale, as illustrated in Fig. 3. In all cases, we obtained only the.“step-type” curves shown, indicating the “immediate” formation of ana on the time scale of our set-up, which could detect transients of above 15 ps half-life. The single exception was a short-lived transient detected at 630 nm (where una does not absorb) but only at -100 “C (c$ Fig. 3(D)). Its intensity was about 10% of that at the peak, but no parallel “growing-in” of the absorption at the peak of the ana form was observed; thus there may be no connection between this signal and the formation of ana. 3.4. Photochromic polymers Their synthesis has been reported [2]. Scheme 3 describes the three types of polymer investigated:
6-
54
p :: P g
2
d O-
Fig. 3. The spectrum of compound C measured by flash photolysis at room temperature (main curve). The insets are kinetic curves taken at -100 “C (A and B at 480 nm and C and D at fX0 nm). At both wavelengths, curves were taken on both short and long time scales, as shown.
A. Zelichenok et al. I Photochemistryand rnrtlfble holographicalrecording in ,vo!ymers
PSX, PS and PMMA. For each type of polymer, we studied the products of incorporation of the photochromic compound B-4 (Scheme 2) through side chains of various lengths (except for PS) and at various concentrations. The polymers were studied both as solutions in THF or toluene, and in the form of films formed by dip coating of glass slides. In each case the rates of the photoconversion fru)t~--,ana were measured, taking into account the absorption at the wavelength used for irradiation, 365 or 405 nm. The absorption spectra were, in all cases, similar to those of solutions of the photochrome. In all films of the photochromic polymers, the reverse reaction ana * trans also takes place, as in solutions of the photochrome proper. The clearest results regarding the kinetics of the fauns *unu phototransformation were obtained for PSX, while for PMMA and PS a substantial scattering of the initial rates of the conversion was observed. Apparently, this stems from a very significant decrease in the degree of polymerization of PMMA and PS with increasing concentration of the active ester monomer in the polymerization mixture [2], since succinimide active ester should be a strong inhibitor of free-radical polymerization. Note that only PSX was obtained by a polymer analogous reaction from a PSX with a given degree of polymerization, 35. In a typical experiment for PSX, a solution of compound C (Scheme 3) in ether, a solution of polymer I, n =8 (Scheme 3), in ether, and a film of this polymer were irradiated at 365 nm under identical conditions inside the sample compartment of the spectrophotometer. The relative initial rates of anu formation were as follows: solution of compound C, 100; solution of polymer, 66; film, 44. Note that the colour decay did not obey firstorder kinetics in the films and the initial rates were calculated from the initial slope of the colourfading curve. The relative rates measured in toluene solutions of compound I, n =8 (Scheme 3), at various con. * ccat:at;ons of the photochrome :n the po!ymer PSX compared with a solution of C, are given in Table 1 (365 nm irradiation). TABLE 1. Relative rates at various photochrome in PSX Photochrome
Molar concentration Relative initial rate
concentrations
Photochrome C 100 (i.e. pure) (%)
10 60
20 60
40 5s
50 47
100
TABLE
2. Effect of n and 7’s on relative rate
n in compound 1 4 8
139
I
Relative rate 100 74 61
initial &) 2 8 38
The effect of the length of the spacer chain in films, as expressed by n in formula I (Scheme 3), is shown in Table 2, in which we also report the corresponding glass transition temperature T,. The concentration was about 30%. Again the effect was not large. T. of the polymer substituted only by C7H15 groups (instead of the photochromic group) was -90 “C. Tg at photochrome concentrations above 10% is almost independent of the concentration, with values of 30-40 “C. The relative rates of frans +una photoconversion at 405 nm of films of the three photochromic polymers I (n = 4), II and III (k = 5)‘and a solution of C in ethzr were a.s follows: PS, 12; PMMA, 25; PSX, 50; solution, 100. The molar concentration of the photochrome was about 12%. Statistical data handling for all polymers with different spacers and concentrations of the photochrome gives a very similar trend for the initial rate: PSX, 7.3 f 1.3; PMMA, 3.3i0.5; PS, 2.5ItO.6. At the highest molar concentration of photochrome in PMMA and in PS, the two polymers are obviously almost identical, because most of the side groups are replaced by the photochrome. 3.5. Holographic recording with the photochromic polymers The possibility has been mentioned of employing 6-phenoxy-5,lZnaphthacene-quinone dissolved in PMMA as a material for holographic recording [l]. In order to examine the feasibility of such an application, we performed the holographic recording on films of PMMA (20% of the photochrome) and PSX (30% of the photochrome). Holographic gratings were written with wavelengths of 413 nm (for tran.r +anu conversionj and 482 nm (for ana --) trans conversion) of Kr + laser and non-destructive read-out with a He-Ne laser at 632.8 nm (for details of the holograph technique, see refs. 9 and 10 and references cited therein). Maximum diffraction efficiencies of 0.2% and 0.5%, were obtained for rrun.s +ana and anu + fruns conversions respectively for both polymers. Kinetics of the hologram growth for PSX and PMMA (writing with 413 nm) are shown in Fig. 4. A rapid
A. Zelichcnok ef al. I Photochemisty
140
0.06 3 e
ii? 0.06
0.6 t
cB 0.04 5 0.03 E “y 0.02 0.01 0.00
’ 0
I
20
40 TIME
BQ IsI
80
100
Fig, 4. Hologram formation at 413 nm (writing beam intensity, 2.0 W cm-l): cutve 1, PSX (30% photochrome); curve 2, PMMA (20% photochrome).
10
0
20
30
40
50
e’o
NCNBER OF CYCLES
Fig, 6. Dependence cording wavelength, PMMA.
of the hologram formation etticiency (re482 nm) on the number of cycles for 20%
0.40
‘;; &
0.30
z E 0.20 E 0
20
40 TIME
80 IsI
80
Fig. 5. Write-erase cycles on 20% PMMA, corresponding to cycles 40-45 in Fig. 6 (au., arbitrary units): curve 1, recording on clna form with 482 nm, intensity of the writing beams of 0.14 W cm-*; curve 2, switching light off; curve 3, erasing with UV tight.
drop to the somewhat lower efficiency could be observed after switching off the writing beam. Holograms recorded on PMMA could be completely erased by irradiation with suitable light, Le. for recording by frans form (413 nm), erasing with visible light, and for recording by ana form (482 nm), erasing with UV light. A number of write (482 nm) and erase (UV light) are shown for PMMA. films in Pig. 5. By contrast, the holograms on PSX films cannot be erased completely by subsequent illumination. Figure 6 shows that the efficiency of holograms recorded on the same spot on a sample of PMMA declined after a number of cycles even though complete erasure by illumination is accomplished each time. The intensity dependence of the hologram formation shown in Fig. 7 indicates that increasing intensity accelerates both the efficiency growth and the photoinduced fatigue of the hologram. The hologram can be fixed by keeping it in NH, vapour over ammonia solution, which converts the
%
0.10
0.00
0
5
10
15
20
TIME IsI Fig. 7. Dependence of the hotogram formation efficiency on intensities of the recording beams (20% PMMA; recording wavelength, 482 nm): curve 1, 14 mW cm-‘; curve 2, 22 mW cm-‘; curve 3, 34 mW cm-$ curve 4, 88 mW cm-‘; curve 5, 140 mW cm-l; curve 6, 280 mW cma2; curve 7, 560 mW cme2.
area form into amino-una-quinone. This procedure increased the diffraction efficiency three times, to about 1.5%. 4. Discussion The main result of the present investigation is that the polymers of all three types in which the phenoxy-naphthacene-quinone was incorporated exhibit qualitatively the same photochromism as the quinone, both in solution and as films. The quantum yield in the films is between 10 and 50% of that in sohrtions of the quinone, probably as a result of the high viscosity of these media. Indeed, the variation in the glass transition temperature T, seems to reflect, to some extent,
A. Zelichcnok et al. i Photochemistry and muIt@le hologruphical wcording in polymers
the variation in the viscosity of the films. The rate of photoconversion decreases with increasing T,, i.e. increasing viscosity, irrespective of the cause of sech an increase (photochrome concentration, length of the (CH& chain, or type of polymer). In solutions of the polymers, the viscosity factor obviously no longer applies. In dilute solutions, the main factor responsible for differences between such solutions and solutions of the quinone proper is the extent to which the photoconversion of the photochromic side group is hindered by the surrounding polymer. From the results obtained, we conclude that this effect amounts to no more than 50% and decreases with increasing length of the chain by which the photochrome is connected to the polymer backbcne. Only at the highest concentration of photochrome does some effect of the interaction between neighbouring photochromic side groups seem to appear in some cases. For the purpose of applications, it is important to note that films with a very high content of photochrome can be prepared, while still retaining most of the mechanical properties of the polymers. Moreover, the insignificant irreversible photochemical side reactions and the absence of thermal interconversion frans +una are of obvious practical advantage. The holographic experiments demonstrated the possibility of erasable recording, in principle, although the low efficiency and insufficient fatigue resistance of the materials represent substantial problems for the application. Finally, reasonable estimates of the absorption sp.Tctrum of the pure ana form and of the quantum yields in both directions for compound C were obtained: about 0.6 and 0.1 for truns dana and ana + tram respscrivtily.
141
Regarding the mechanism of the photoconversion, the fact that rates of photoconversion are not affected by oxygen indicates that a triplet transient is not involved. The flash photolysis results show that, even at -100 “C, the photoformation of the mu modification in solution is instantaneous on the time scale of the equipment used, i.e. about 15 p. Of course, this does not rule out the existence of shorter-lived transients between singlet-excited truns and ground-state ana. Acknowledgment Support from the German-Israeli Foundation for Scientific Research and Development is gratefully acknowledged. References A.V. El’tsov (ed.), OtgunicPhotochromes, Consultants Bureau, New York. 1990, p_ 210. F. Buchholtz, A. Zeiichenok and V. Krongauz, Macromolecules,
26 (1993) 906. E. Fischer, Mol. Photochem., 2 (1970) 99. E. Fischer, Mol. Photochenz., 6 (1974) 1. H.G. Helter and I.R. Langan. EPA NW.& (October 1981) 71. IUPACCommission on Photochemistry, ChemicalAcfinometry, 1988, in Pure Appl Chem., 61 (1989) 187. K. Macda and E. Fischer, Helv. Chim. Actu, 66 (1983) 1961, and references cited therein. Yu.E. Gerasimenko, A-A. Parshutkin, N.T. Poteleschenko, V.P. Poteleschenko and V.V. Romanov, Zh. Priki. Spectrosk, 30 (1979) 954. 9 J. Pinsl, M. Gehrtz and C. Brluch1e.J. Phys. Chem., 90 (1986)
6754. 10 C. Briiuchle and H. Anneszr, Holographic spectroscopy and holographic information recording in polymer matrices, in J.P. Fouassier and J.F. Rabek (eds.), Losers in Polymer Science and Tcchnolow: AppIicafions, CRC Press, Boca Raton, FL, 1990.