Polysilylenes thin films

The only other polysilylene for which morphological information has been obtained so far is PDBS. Under similar conditions of thin-film growth as described for PDHS, much smaller crystallites are obtained (Figure 28a). Their lengths are in the order of 0.2 (xm, and their thicknesses are 40-70 nm. This reduced thickness (compared with that of PDHS) is consistent... 

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. 

Figure 6 displays the phosphorescence spectra of three alkyl polysilylenes obtained from pulsed laser excitation of thin film samples at low temperature. For these spectra, the data was integrated over the period from 200 pseo to 0.01 sec following the laser pulse. All of the alkyl polysilylenes studied show a substantial amount of emission in the region of fluorescence which has essentially the same shape as the normal fluorescence. We interpret this emission as delayed fluorescence resulting from triplet-triplet annihilation. The delayed fluorescence provides a convenient comparison between the fluorescence shape and width and that of the phosphorescence. 

Figure 5. Thin film thermochromism for several polysilylenes.

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. ... 

The effect of main chain crystallization was not studied because of our inability to prepare coherent thin films of highly crystalline polysilylenes such as polydimethylsilylene or polydiethylsilylene. 

---