J. MATER. CHEM., 1991, 1(3), 481-482 48 1 Photoinduced Reversible Refractive-index Changes in Tailored Siloxane-based Polymers Susan H. Barley, Andrew Gilbert and Geoffrey R. Mitchell* Polymer Science Centre, University of Reading, Whiteknights, Reading RG6 ZAF; UK The design and synthesis of novel polymers for use in optical-fibre space switching devices is described. These materials consist of a low refractive index matrix with a small fraction of photoactive units together with adjuster units to tailor the refractive index to that of silica. These materials show reversible and controlled variation in the refractive index which is induced through selective irradiation at either 366 or 525 nm. Keywords: Photoactive polymer; Optical communication; Hydrosilation ; Refractive index At present, voice, computer and other electronic information is transferred at high speeds and densities over large distances using high-bandwidth optical-fibre systems.One of the factors which governs the more general application of optical trans- mission is the ability to switch and route signals without the need for repeated conversion to and from the electronic domain. It is clear that there are considerable advantages to be gained if the switching processes could be made without such transformations, in other words if the switching were performed optically. One particular possibility associated with optical fibres is the construction of switching devices directly involving the optical fibres, in which the transfer is made by coupling the evanescent wave in one fibre into a second fibre.' A number of methods have been employed in attempts to produce switches based on this process.'-' An intermediate material may be placed between the two cores such that when the refractive index of that material is low no transfer takes place; in effect the intermediate layer acts as the cladding.However, if the refractive index is raised then coupling or switching is achieved. A relatively small change in refractive index is required to activate the switching mechanism. In this contribution we report the design, synthesis and characteristics of novel polymeric materials for use in a system in which the heart of the optical-fibre switch is stimulated using light.The most basic problem of designing a suitable material for such a switch is that the refractive index of the material must more or less match that of the optical-fibre core. For conventional telecommunication optical fibres this is ca. 1.46. Most 'active' materials such as liquid crystals or non-linear optical films have inherently high refractive indices. In the materials described here this problem is circumvented by utilising a material whose matrix has a low refractive index, into which are introduced a small proportion of photoactive groups. These photoactive groups undergo reversible cis-trans isomerisation, and this allows the photoactive units to be optically pumped almost exclusively from the trans to cis states and vice versa.In addition to the change in optical absorption characteristics and hence refractive index, the trans and cis isomers have different molecular shapes. These induced geometric changes to the photoactive groups may invoke variations in the optical properties of the matrix material, for example through the effective bulk density. The materials described here, therefore, consist of a polymer backbone, which will give rise to a low-refractive-index material [in our case poly(dimethylsiloxane)], photoactive units, which in this example are azobenzene derivatives, and an 'adjuster' unit, by which the overall refractive index is tailored to match that of silica, and these are dodecane moieties. Poly(dimethylsi1ox- ane) itself has a refractive index of ca.1.399-1.4035* and a remarkably low glass-transition temperature, two attributes which make it particularly suitable for the base material in these devices. The reversible photoisomerisation of azobenzene units is well establishedg although we have considered the use of alternative photoactive units." The attachment of the photoactive units to the siloxane backbone was performed using the Pt-catalysed hydrosilation reaction."-12 The base polymer poly(methylsi1oxane) was reacted with 4-(allylo~y)azobenzene'~and dodec- I-ene in varying amounts to produce a range of polymers with con- trolled refractive indices and differing levels of azobenzene (Scheme 1). The base polymer poly(methylsi1oxane) (PMS) with Mw4500-5000 was used as supplied from Petrarch.To a solution of PHS (5 g, 0.105 mmol) in toluene (220cm3) 4-(allyloxy)azobenzene and dode-1 -ene were added in the required ratios followed by a freshly prepared solution of H2PtC1, in isopropyl alcohol (0.01 mol dm-3, SiH :Pt ratio of 1 :5 x This mixture was heated under nitrogen at 50-60 "C for 1 h followed by refluxing until no sign of the Si-H absorption peak (2155 cm-') could be seen by infrared spectroscopy. Typical yields for the hydrosilation were 50%. In order to determine the photoinduced refractive-index changes, thin films of each polymer were cast from dichloro- methane solution directly onto the prism face of an Abbe I I CH3-Si-0 Si-0 Si-CH3 ICH3 + + CH2=CH--(CH2)g-CH, Scheme 1 482 Refractometer.These thin films were irradiated using a wave- length band of light of 16 nm width centred at 366nm to pump from the trans to the cis states and at 525 nm to reverse the transition. The light source was a Xe/Hg broad-band arc lamp (200 W) equipped with an elliptical mirror (Photon Technology International) illuminating the slit of a grating monochromator (Jobin Yvon H 1OUV) through a water-based infrared filter. The fraction of the cis isomer was determined, after removal of a small portion of the film onto a quartz substrate, using a UV-VIS spectrometer (Perkin-Elmer 330). The method employed a calibration curve, constructed using the spectra recorded for solutions of 4-(allyloxy)azobenzene which had been characterised using high-performance liquid chromatography. l4 Typically, over the composition range the errors associated with the determination of the cis fraction were 15% and those with refractive-index measurements were 3 x 104.The temperature coefficient of a polymer with q =0.05 was 3 x K-'. The thermally driven cis+trans conversion was sufficiently slow (time to 50% conversion ca.20 h at 16 "C) to facilitate these measurements after illumination. Fig. 1 shows the variation induced in the refractive index through selective irradiation in a siloxane polymer with q= 0.05 and its relationship to the fraction of the cis isomer in the sample. This curve demonstrates the clear link between the isomeric state of the photoactive unit and the refractive index.The system is photoreversible and Fig. 1 should be seen as representing compositions obtained by both the trans+cis and cis+ trans reactions. The latter reaction is achieved, albeit at a slower rate than the former, by optical pumping at 525 nm. For the polymeric films maximum refrac- tive-index change was reached after ca. 10-20 min with an irradiation power of ca. 0.12 mW cm-2. Fig. 2 shows the effect of changing the levels of substitution of the photoactive units upon the maximum refractive index that can be achieved through the photoinduced changes described above. Clearly, as the level of photoactive units varies so does the concentration of dodecyl units, since within the limits of detection of free Si-H units, p+ q =1 (Scheme 1): this feature leads to variation in the base refractive index.As a consequence, the results are plotted as the deviation induced from the base figure. Over the range of substitution considered here the base refractive index varies from 1.4574 to 1.4888 illustrating the manner by which the final polymer may be tailored to suit a particular application. The most striking feature of Fig. 2 is that relatively small levels of the 1.466 1.465 0 X U .-$ 1.464.-U w *E 0E 1.463 U I I I 1 I I I I 11.462 0 0.2 0,4 0.6 0.8 1 fraction of cis isomer Fig. 1 A plot of measured refractive index at 22 "C of a siloxane- based polymer containing azobenzene units with q =0.05 against the fraction of the cis isomer of the azobenzene unit in that polymer.The series of compositions was prepared by successive irradiation of the polymer with a narrow wavelength band of light centred on 366 nm J. MATER. CHEM., 1991, VOL. 1 0.0p I -0.5 0 7 ;-2.01 .-C 0 03 c2 -3.0 0 0 L I I I-3.51 I 0.0 0.1 0.2 fraction of photoactive units Fig. 2 A plot of the maximum refractive-index change induced through selective irradiation of a series of azobenzene-containing siloxane-based polymers in which the fraction q of the photoactive units is varied photoactive unit are required to give significant and useful changes which may be exploited in an optical space switch.I5 In fact, increasing the level of the photoactive units appears to result in a slight decrease in effectiveness and of course raises the base index above that of silica.An equivalent system in which the unbound chromophore was dissolved in toluene showed refractive-index changes linear with chromophore composition.16 In all cases the changes induced are reversible either through optical pumping or through thermal excitation. In practice, the photoinduced refractive-index changes are relatively stable with time because of the slow thermally driven cis-trans conversion. In summary we have designed and synthesised some novel siloxane-based materials with tailored low base refractive indices, in which the refractive index may be controlled optically in a reversible and significant manner. It is possible to construct from these photoactive polymers optically stimu- lated optical-fibre s~itches'~~" with potential use in optical switching and processing systems.This work was funded by British Telecom. References 1 M. F. Digonnet and H. J. Shaw, IEEE J. Quant. Electron., 1982, QE18, 746. 2 C. Dahne and A. Harmer, Electron. Lett., 1980, 16, 674. 3 P. Yennadhiou and S. Cassidy, Electron. Lett., 1987, 23, 1385. 4 N. J. Moll and D. Dolfi, App. Optics, 1983, 22, 2944. 5 M. B. J. Diemeer and W. J. De Vries, Electron Lett, 1988,24,457. 6 E. S. Goldburt and P. St. J. Russell, Appl. Phys. Lett., 1985, 46, 338. 7 S. A. Cassidy and P. Yennadhiou, ECOCILAN, 1988,88,43. 8 R. Anderson, R. Arkles and G. L. Larson, Silicon Compounds Register and Review, Petrarch Systems, 1987. 9 J. Griffiths, Colour and Constitution of Organic Molecules, Aca-demic Press, London, 1976. 10 S. H. Barley, A. Gilbert and G. R. Mitchell, Makromol. Chem., 1991, in the press. I1 A. Hajaiej, X.Coqueret, A. Lablache-Combier and C. Loucheuz, Makromol. Chem., 1989,190, 327. 12 G. Nestor, M. S. White, G. W. Gray, D. Lacey and K. J. Toyne,Makromol. Chem., 1987, 188, 2759. 13 A. Shukurov, S. D. Nasirdinov, A. G. Makhsumov and N. N. Edganov, Zh. Obschch. Kim., 1986, 56, 2579. 14 A. Proctor, A. Gilbert and G. R. Mitchell, Makromol. Chem., submitted. 15 G. R. Mitchell, S. Cassidy, S. H. Barley and A. Gilbert, to be submitted. 16 S. H. Barley, A. Gilbert and G. R. Mitchell, to be submitted. 17 UK Pat. Application UK9014445.2. Communication 0105515H; Received 7th December, 1990