Substituted polysilylenes

Kepler et al. (1983, 1984, 1987) measured hole mobilities of poly(methyl-phenylsilylene) (PMPS). The mobilities were field dependent and thermally activated with an activation energy of 0.24 eV. The transport was nondispersive at room temperature. With decreasing temperature, however, the transients become increasingly dispersive. Because the same mobility was observed in a wide range of alkyl-substituted polysilylenes, Kepler et al. argued that hole transport occurs through the conjugated states of the chain backbone. 

Soluble disubstituted polysilylenes are a class of polymers that recently has generated great interest. These polymers have the structure [-SiRR -] , in which R and R may be aryl or alkyl groups and R may be the same as R. The substituted polysilylenes exhibit a wide variety of physical properties, depending on the nature of R and R. Of particular interest is their intense UV absorption at 300-400 nm both in solution and in the solid state, a property conferred by the silicon backbone and accompanied... 

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... 

For the substituted polysilylenes, (SiRR ) , the coupling constant can be varied systematically by changing the side groups (this change affects e and Vd via the backbone polarizability) or the solvent (this change affects Vj) via the London dispersion forces e is expected to be only weakly solvent dependent for nonpolar systems). Therefore, in principle, the three distinct phase behaviors predicted by the theory may be observed by judicious choice of polymer-solvent pairs. 

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 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... 

Another class of soluble polysilylenes exhibits essentially no or very weak thermochromism. This class includes poly(cyclohexylmethyl- 15, 38), poly(phenylmethyl- 15, 38), (polytrimethylsilylmethyl- 15), and poly(diarylsilylenes) 46), all of which appear to be conformationally locked over a wide range of temperatures. In terms of our theoretical perspective, this behavior would arise from the steric effects of bulky substituents, which imply a large value of e and, hence, a small coupling constant Vj /e. For aryl-substituted polysilylenes, the proximity of an aromatic group to the backbone could also stabilize a highly ordered rodlike conformation via enhanced dispersion interactions. 

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. 

We have previously suggested (9a, 1 0) a rotational isomeric state model to explain the solution thermochromism exhibited by the un-branched alkyl substituted polysilylenes. This model treats the absorption spectrum as a superposition of the spectra of Isolated... 

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