Is a small bandgap thiophene polymer attainable

The S(k) contributions nearly cancel for the HOCO and LUCO relative to the HOCO and LUCO of PA. 

The gap is therefore entirely determined by the BLA value of the PA backbone. This being reduced, the gap is reduced relative to the 1.5 eV gap of PA. 

In the rest of this section, we discuss examples along the A-Q copolymer route to pursue intrinsic low bandgap polymers through various A-Q combinations. A few new results will be included in the tables and we have also re-done calculations using the same level of theory throughout so that we can discuss comparable data. 

In an A-Q copolymer, we could use any A unit similar to those listed in Table 7.3 and any Q unit similar to those in Table 7.4. Consequently, a large number of combinations and derivatives may be pursued. Some of the experimentally available A-Q copolymers are listed in Table 7.5 together with their theoretical and experimental bandgaps. Several groups have considered poly(ABA) copolymers and therefore both poly(AB) and poly(ABA) are included. 

It is interesting that earlier calculations based on the geometry-optimized LHS semiempirical MO theory on the series shown in Table 7.6 provided a decreasing trend for the gap across the series, because all three appear as aromatic at that level of theory (Ktirti et al 1993). The level crossing does not emerge yet in the first three members of the series. This indicates two general points worth considering (1) the 

---