Polysilylenes transitions

Absorption spectra displaying a thermochromic transition have also been reported for several of the symmetrically substituted polysilylenes in the solid state. One example is that of PDHS, as reported by Kuzmany et al. (9) and shown in Figure 6. At 45 °C, a single absorption at 317 nm is observed, which is very similar to the absorption maximum for this polymer in solution... 

As the sample is cooled, a second absorption band is observed in the range from 365 to 375 nm, which continues to grow upon further cooling. This behavior is completely reversible. This type of thermochromic transition is not observed in the solid-state absorption spectra of PDBS (iO) or PDPS 10,11). To understand this unusual absorption behavior of the polysilylenes in solution and in the solid state, a variety of studies have been directed toward the determination of the polymer chain conformation. 

PDHS Structures in Solution. The determination of the chain conformation of polysilylenes in solution, particularly the conformations at temperatures just above or below the low-temperature thermochromic transition, is of great interest. NMR spectroscopy is one of the most useful techniques for probing chain conformation in solution (2i), and NMR is especially effective because of the large sensitivity of the carbon chemical shift to bond conformation (22). Silicon nuclei are also very sensitive to chain conformation, but a good correlation between silicon chemical shift and bond conformation has not been established yet. Unfortunately, both of these nuclei suffer from low sensitivity, primarily because of their low natural abundance. In contrast, protons have an essentially 100% natural abundance, but compared with the carbon or silicon chemical shift, the proton chemical shift is not very sensitive to bond conformation. Efforts to use NMR to probe the low-temperature dilute-solution conformation of the polysilylenes have been unsuccessful thus far. The diflSculty is that PDBS and PDHS precipitate from solution in 20-30 min after cooling through the thermochromic tran-... 

Other Polysilylenes. The symmetrically substituted poly(di-n-tetra-decylsilylene) is reported to have a TGTG trans-gauche-trans-gauche ) conformation, and a bathochromic shift is observed in the UV spectrum at 54 °G with Xjnax shifting from 322 to 350 nm (35). The structures of many polysilylenes may be more complicated than what have been discussed thus far. As an example, the DSG data for PMHS are shown in Figure 24, The figure shows that Tg (glass transition temperature) is 220 K and that two... 

Although much has been learned about the structures of polysilylenes, a tremendous amount of work remains before a full understanding of these materials is developed. The microstructure of the polymers can be studied directly by solution NMR spectroscopic techniques. The determination of the chain conformation in solution is diflScult, particularly at low temperature. Light-scattering techniques may be able to establish the solution dimensions of the polysilylenes through the low-temperature thermochromic transition. The chain conformation in the solid state can be established by X-ray and electron difiraction methods. Solid-state Si NMR spectroscopy can become... 

Order-Disorder Transitions and Thermochromism of Polysilylenes in Solution... 

Order-disorder, or rod-to-coil , transitions in dilute solution have been reported for polydiacetylenes (2, 5-11), polysilylenes (12-15), and alkyl-substituted polythiophenes (16). The interpretation of the experimental observations has been the subject of considerable controversy with respect to whether the observations represent a single-polymer-molecule phenomenon or a many-chain aggregation or precipitation process (3-16). Our own experimental evidence (12, 13) and that of others (5-8, 10, 16) weigh heavily in favor of the single-chain interpretation. In our theoretical interpretation, we will assume that the order-disorder transitions observed in dilute pol-ysilylene solutions represent equilibrium, single-chain phenomena. 

In this chapter, the theory of conformation-dependent polymer-solvent interactions, which was developed in detail by Schweizer (20-22) for soluble TT-conjugated polymers, will be used to explain both qualitatively and quantitatively a large body of observations on the polysilylenes (23, 24). The same theory has been used recently to interpret qualitatively order-disorder phenomena and the electronic thermochromism of TT-conjugated-polymer solutions and films (25, 26). The study presented in this chapter represents part of an ongoing effort to understand in a unified fashion both the optical properties (27-30) and order-disorder transitions (20-24) of flexible, conjugated-polymer solutions. 

Order-Disorder Transitions. General Features, Experimental data are summarized in Table II, and representative thermochromic behaviors are shown in Figure 2. For the dialkyl-substituted polysilylenes the transition is very sharp, with a barely discernible coexistence region and an approximate isosbestic point. On the other hand, the asymmetrically substituted polymers, except poly(n-dodecylmethylsilylene), display very smooth behavior only in n-hexane solution and a broad but clearly discernible transition in dilute toluene solution. The transition width (ATc) in toluene solution was taken to be the interval between departure from the extrapolated, smooth, high-temperature behavior and the onset of peak absorption wavelength saturation at low temperature. The transition temperature (Tq) is defined arbitrarily as the midpoint of this region. 

Table II. Order-Disorder Transition Temperatures and Widths of High-Molecular-Weight Polysilylenes ...

Table I, estimates of Vp/e relative to the corresponding value for poly(di-n-butylsilylene) were obtained poly(di-n-hexylsilylene), 0.91 poly(n-propyl-inethylsilylene), 0.61 poly(n-hexylmethylsilylene), 0.59 and poly(n-dodecylmethylsilylene), 0.44. Therefore, although the absolute magnitude of Vp/e cannot be calculated accurately, the theory is fully consistent with the observation of abrupt transitions (strong coupling) for the dialkyl-substituted polysilylenes and smeared or nonexistent transitions (intermediate or weak coupling) for the atactic polysilylenes.

Symmetrical Dialkyl-Substituted Polysilylenes Because of their extremely sharp order-disorder transitions, the nonpolar, symmetrical dialkyl-substituted polysilylenes are almost ideal systems with which to test the predictions discussed earlier. The predicted solvent dependence of Tc was tested by performing a series of experiments with high-molecular-weight poly(di-n-hexylsilylene) in dilute solution. Experimental results for six solvents are listed in Table II, and the theoretically defined solvation coupling constants and solvent parameters are collected in Table III. 

The predicted intrinsic width of the order-disorder transition of a mono-disperse, flnite-molecular-weight polymer solution was also tested. The average molecular weights of dialkyl-substituted polysilylenes are in the order of 6 X 10, which implies that N is 3000-5000 silicon atoms. With equation 9, the theory predicts that ATq/Tc is 0.004-0,006, which for Tc = -30 corresponds to an intrinsic width of roughly 1 or 2 C. This result is in good agreement with the experimental observations summarized in Table II. 

Unsymmetrical Alkyl-Substituted Polysilylenes A detailed comparison of theoretical predictions and experimental results for the atactic polysil-ylenes is more diflScult for several reasons (1) the observed transitions are much broader, (2) the effects of random substitutional disorder are not included in the theory, and (3) the magnitudes of the consequences of stereochemical disorder are expected to vary for different atactic polymers. Nevertheless, for all the asymmetrically substituted polysilylenes studied, except poly(n-dodecylmethylsilylene), the predictions discussed earlier... 

The higher transition temperature of poly(n-propylmethylsilylene) in toluene compared with that of poly(n-hexylmethylsilylene) is especially significant, because these two polysilylenes have very similar free energies of defect formation (Table I), and poly(n-propylmethylsilylene) is characterized by a larger less side-chain screening of the backbone from toluene. 

Finally, solid films of some polysilylenes exhibit thermochromism and undergo true thermodynamic order-disorder phase transitions at much higher temperatures 18, 19, 47, 48) than in solution (typically, Tq 40-80 °C). In the context of the theory, a larger refractive index of the neat solid compared with that of the dilute solution results in a higher predicted Tq 21). However, we do not believe that the observed high TqS in films can be explained solely by this effect. Previous explanations have been made exclusively in terms of side-chain crystallization 18, 19, 48). Packing effects should be more important in the solid state, but both intramolecular and intermolecular packing effects must be carefully considered. Indeed, the fact... 

We have studied the thermochromism of fluorescence and show this behavior to be consistent with the rotational isomeric state model previously proposed to explain solution thermochromism in absorption (9,10). Weak, structured phosphorescence is observed from all polymers studied. The contrast between the structured phosphorescence and the narrow fluorescence is interpreted as evidence that the triplet state is the immediate precursor to photochemistry. Finally, the change in the fluorescence character in the aryl series on going from phenyl substitution to naphthyl substitution suggests a change in the nature of the transition from one involving mixed side chain-backbone states in the phenyl case to one which is primarily side chain-like for naphthyl-substituted polysilylenes. 

In all of the polysilylenes studied, the fluorescence from neat thin films on fused silica substrates exhibits a blue shift upon cooling. In cases where our studies have spanned the glass transition of the polymer, no change in behavior is seen (Figure 4). In the polymers which have substantial crystallinity, an abrupt shift in behavior occurs at the crystalline melting point above this temperature the films behave in much the same fashion as the fluid solutions. These phenomena have been extensively studied (9,12,13) and will not be treated here. 

We have examined the emission spectra of a variety of polysilylenes as thin films and solutions. The solution fluorescence ther-mochromism provides evidence to support the rotational isomeric state model used to interpret the absorption spectrum. The structured character and low yield of phosphorescence in the alkyl polysilylenes suggest that the triplet is the immediate precursor to photochemical scission. The change in character of both fluorescence and phosphorescence on progressing from phenyl to naphthyl in the aryl series indicates that the transitions in the naphthyl polymers are principally ir—it. ... 

Recently, several new methods of preparing polysilylenes have been reported. Transition metal catalyzed dehydro-genative coupling of hydrosilanes, especially of RSiHa, is... 

Polysilanes (or polysilylenes) are usually prepared by the Wurtz-type couphng reaction of dichlorodialkylsilanes or, alternatively, via a transition metal-catalyzed dehydrogenation of dialkylsilanes both approaches often exhibit difficulties in terms of controlling the molecular weight and chain-structure, however. Nonetheless, by using masked silylene monomers (l-phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes), a series of novel, well-defined linear poly(silylene)s (M /Mn 1.3) was successfully obtained via an anionic ROP (Scheme 5.11) [147-150]. 

Effect of Thermally Induced Transitions on Electronic Transport in Aliphatic Polysilylenes... 

In this chapter we describe results of TOF measurements on aliphatic polysilylenes and poly(di-n-butylgermylene) carried out over a broad range of temperature encompassing the glass transition temperature Tg, and the phase I - phase II (order - disorder) transition caused by side chain ordering and melting, which is accompanied by a blue shift in the uv spectra of the polymers. 

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