Increased spacer length

The variations on the spacer module within 29 are available from Dabes-tani s laboratory [89]. The methylene spacer has been systematically extended to the tetramethylene case. As may be anticipated from previous studies on structurally similar aminoalkylaromatic PET systems [90-92], the fluorescence quantum yield (4>f) of the ion-free sensor rises gradually with increasing spacer length as the PET rate drops. Potassium ions cause moderate to small fluorescence enhancements, but the same trend can be discerned in the values, though muted. As usual, protons cause the largest enhancements of all and the ([>f values remain constant across the series since PET has been completely killed off. 

The pH dependent photophysics of phenylalkylamines (5) [8] illustrates the features that simple molecules can provide during mechanistic investigations in a new field. The fluorescence quantum yields (F) and lifetimes (tf) of (5, n = 1,2 and 3) when protonated are similar to those of the parent fluorophore, toluene. However, differences appear as the pH is increased. The values of the proton-free (5) (mia) allow the ranking n = 2>n = 3>n=l, which is also supported by the lifetime data. The unique importance of folded conformations in (5, n = 3) rationalizes this deviation from the simple expectation of decreasing PET rate with increasing spacer length. 

As a result of their low Tg values and lack of crystallinity, many of these polymers showed liquid crystallinity at room temperature. The liquid crystalline mesophases of the monomers were identified as nematic but the polymeric mesophases were not identified, although they possessed a very broad thermal stability between their Tg and their clearing transitions. A mesophase temperature stability of up to 170 °C was observed for the polymers with bicyclohexane central mesogenic units. These polymers showed decreases in Tg and Tj with increased spacer lengths. 

In the special case of side-chain LCPs, it was found that the smectic order extends over about 1000 A [24], This value increases with temperature which was interpreted by the annealing of defects. The correlation length also increases with increasing spacer length which means that the smectic order develops more easily when the coupling between the mesogenic cores and the backbone gets weaker. 

Within each series, the isotropization entropies for homologs with the same parity lie on smooth curves which increase with increasing spacer length, even members giving the upper curve. The isotropization entropies of the members of the series with n = 2 and 4 containing the same number m of methylene units are very similar to each other, but significantly lower than those of the corresponding members of the series with = 3. In addition, the slope of the iso-... 

Figure 16 shows the change in enthalpy and entropy as a function of the length of the spacer for the three polymer series plotted in Fig. 14. It demonstrates that both the change in enthalpy and entropy of isotropization from the nematic and s mesophases increase linearly with increasing spacer length. Although such increases in AH, and AS, have been attributed to more efficient deeoupling of the mesogen from the polymer backbone and therefore to inereased order [231], it simply corresponds to a con-...

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