Sensitivity to Side Chain Structure

There are a multitude of secondary effects directly related to the side chain placement and chemical architecture which have a tremendous impact on the final structural ordering and phase behavior. For example, in P3AT homopolymer hosts it has been observed that varying degrees regioregularity, with respect to the side chain chemical binding site, can alter many physical properties. Monosubstituted polythiophenes, containing an average of one alkyl chain per thiophene ring, often exist in a trans-planar main chain conformation which has a doubled monomer unit c-axis repeat of $\sim$7.8Å. There are three possible monomer to monomer alkylthiophene couplings, head-to-tail (HT), head-to-head (HH) and tail-to-tail (TT) as shown in Fig. 17. Depending on the specific synthetic approach the regioselectivity, with respect to only the percentage of HT configurations, can span the full range of 0% to nearly 100%. At HT regioselectivities of close to 0%, there are considerable steric interactions which frustrate both the formation of a planar main chain conformation and of the lamellar construction[114]. Initial studies[127] of poly(4-4'-didecyl-2,2'-bithiophene) (PDOBT), with an all [-(HH-TT)-]_n repeat, indicate 40$^\circ$ torsional angles between adjacent of thiophene rings. Unlike the trans-planar P3AT's discussed above, PBOBT is not extensively $\pi$-conjugated. Therefore it does not exhibit thermochromism. Surprisingly cast film of this polymer can still exhibit a fairly well ordered lamellar phase. HT regioselectivities ranging near 75% enable a high degree of structure ordering in this phase and are thermochromic but yield only modest conductivities (after doping). Highly regioselective samples[7], 90% and above, yield similar structures but sharply enhance the measured conductivities. Moreover there is some evidence that these highly regular P3AT's can be prepared in metastable states in which there is nearly full interdigitation between alkyl side chains from immediately adjacent stacks[].

 

Figure 17: Schematic of three types of regioisomers for the 3-alkyl substituted thiophenes.

In addition to regioregularity, the synthetic method can be adjusted to yield a variety of polyalkylthiophene ``copolymers''[110]. In these host systems the side chain substation is modified to incorporate two or more side chains units using various isomers or units of differing lengths. The resulting copolymers may be at random or highly regular. The structural response is also intriguing[127]. Stereoirregular alkylthiophene(A)-thiophene(T), -(A-T)-(A-T)-, or -(T-A-T)-(T-A-T)- polymers typically exhibit relatively poorly ordered lamellar structures whereas stereoregular -(A-T-A)-(A-T-A)- polymer exhibit reasonably well-ordered structures. In some of these cases it is possible to simultaneously have a trans-planar backbone conformation and a poorly ordered lamellar structure. The PPV side chain derivative shown in Fig. 13(b) is tailored for the expressed purpose to suppress interchain molecular ordering by having two very different side chain lengths including one of which is branched. There is clearly a very subtle interplay at the molecular level which gives rise to this disparity of manifested structure-properties relationships.