Ionochromism and Increase in Fluorescence Quantum Yield of an Ether-Substituted Polysilylene upon Adding Lithium Ions in Solution Seiji Toyoda † and Michiya Fujiki* NTT Basic Research Laboratories, 3-1 Morinosato, Wakamiya, Atsugi, Kanagawa 243-0198, Japan Chien-Hua Yuan ‡ and Robert West* Organosilicon Research Center, Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706 Received September 28, 1999 Revised Manuscript Received January 19, 2000 Introduction. Polysilylenes show intense absorption and fluorescence bands in the ultraviolet region, as- cribed to electronic transitions involving extensively delocalized σ-electrons along the Si backbone. 1 In many polysilylenes the electronic absorption is thermochro- mic, 2,3 reflecting temperature-dependent changes in the conformation of the main polymer chain. In special cases other kinds of chromotropic behavior have been ob- served, including electrochromism, 4 piezochromism, 5 and solvatochromism. 6,7 Recently, solid films of polysi- lylenes containing ethyenedioxy units in the side chains have been shown to display ionochromism. 8 Upon ad- dition of lithium salts, the thermochromic frequency shift for such polymers may either be inhibited (negative ionochromism) or accelerated (positive ionochromism). 9 Here, we report positive ionochromic behavior of a polysilylene in solution, affecting both the absorption and fluorescence spectrum. Experimental Section. A. Synthesis. Sodium 2-ethoxyethoxide (165 g) in toluene (100 mL) was slowly added to 1,5-dichloroethane (254 g) at 5 °C. 1-Chloro- 5-(2-ethoxyethoxy)pentane was isolated by distilling the reaction mixture under vacuum. 13 C NMR in CDCl 3 : 14.28, 23.38, 28.91, 32.08, 44.91, 66.80, 68.20, 69.81, 70.25. The chloride (50 g) was reacted with magnesium (3 g) at 75 °C. The mixture was added slowly to tetrachlo- rosilane (18 g) in dry diethyl ether at 40 °C. Di-6,9- dioxaundecyldichrolosilane (1) was isolated by distilling the reaction mixture in a vacuum. 13 C NMR in CDCl 3 : 15.10, 24.38, 29.91, 32.88, 45.91, 67.90, 68.80, 70.51, 71.25. 29 Si NMR in CDCl 3 : 32.08. The monomer 1 (3 g) was reacted with sodium metal (0.5 g) in toluene (30 mL) for 30 min at 110 °C. After removal of toluene from the mixture in a vacuum, the resulting solids were dissolved in ethanol (100 mL) and filtered under an Ar gas pressure. A small amount of water was added to the filtrate, and the precipitate was isolated using a centrifuge apparatus. After drying the polymer at 100 °C overnight under reduced pres- sure, the purified poly[bis(ethoxyethoxypentyl)silylene] (PEEPS) was obtained as a white solid. The weight- average molecular weight of the polymer (M w ) was 2.2 × 10 6 , and the polydispersity index was 2.5, based on a calibration with polystyrene standards. B. Spectroscopy. UV absorption and fluorescence spectra were measured with band-pass of 1 nm. The fluorescence quantum yield (Q-FL) was determined using quinine sulfate as a standard. Results and Discussion. Figure 1 shows the ab- sorption spectra of PEEPS in dilute di-n-butyl ether at 23 °C when lithium trifluoromethanesulfonate (LiT- FMS) solution was added incrementally to the solution. At a LiTFMS:Si ratio of 200 (in moles), the absorption band was slightly red-shifted. Little further change took place up to a LiTFMS:Si molar ratio of 800; but when the ratio was increased to 1000, an abrupt shift of the absorption band from 325 to 350 nm occurred, along with significant band narrowing. The bathochromic shift suggests the formation of a more extended (transoid) conformation for the polysilylene chain, 1-3 and the nar- rowing of the band is consistent with an increase in the segment length. 10 We consider that the iono- chromism results from a noncovalent interaction be- tween the Li + cations of the LiTFMS and the oxygen atoms of the substituents, which induces a conforma- tional change in the polymer to a more extended, transoid arrangement. The samples containing LiTFMS also showed absorp- tion at short wavelengths, <280 nm, which may be due to light scattering by aggregates of PEEPS with Li + . Filtration of the solutions containing high concentra- tions of LiTFMS through a 0.45 μm filter removed all of the polymer from the solution, confirming that aggregation had taken place. The fluorescence spectra of PEEPS as a function of LiTFMS concentration are shown in Figure 2. At a LiTFMS:Si molar ratio of 200, the fluoresence band was red-shifted from 344 to 354 nm. Little further change took place up to a LiTFMS:Si ratio in moles of 800. When the ratio was increased to 1000, the fluorescence band shifted abruptly from 354 to 359 nm. The Si unit concentration was 7.14 × 10 -6 M at a LiTFMS:Si molar ratio of 1000. The full width at half-maximum of the fluorescence spectrum was 21 nm for the pure polymer, 17 nm at a LiTFMS:Si molar ratio of 200, and 14 nm at a LiTFMS:Si ratio of 2000. Thus, the spectral change of the fluorescence bands upon adding the salt was somewhat different from that of the absorption bands. When a solution of PEEPS containing lithium salt at a LiTFMS:Si molar ratio of 1000:1 was diluted with solvent, the UV absorption spectrum was unchanged; no band at 325 nm for uncomplexed polymer was observed. When more polymer was added to a solution at the same 1000:1 LiTFMS:Si molar ratio, two bands at 325 and 350 nm were observed, without any weaken- ing of the 350 nm band. These results indicate that the complexation is irreversible, at least at room tempera- ture. The polysilylene absorption spectrum results from a convolution of various segmented chromophores with different excitation energy. 11 When segments with higher energy absorption are excited, the photoexcited electron-hole pair migrates into segments with lower excitation energy, from which the emission occurs. The fluorescence spectra for polysilylenes strongly depends on emission from these segments with lower energy † Present address: NTT Photonics Laboratories, 162 Tokai, Naka-gun, Ibaraki 319-1193, Japan. ‡ Present address: Materials Research Laboratories, Industrial Technology Research Institute, Hsinch, Taiwan 31015, Republic of China. 1503 Macromolecules 2000, 33, 1503-1504 10.1021/ma9916483 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/10/2000