  There is currently a great need for solid state lasers that emit in the
infrared, as this is the operating wavelength regime for applications in
telecommunications. Existing $\pi$--conjugated polymers all emit in the visible
or ultraviolet, and whether or not $\pi$--conjugated polymers that emit in the
infrared can be designed is an interesting challenge. On the one hand, the
excited state ordering in trans-polyacetylene, the $\pi$--conjugated polymer
with relatively small optical gap, is not conducive to light emission because
of electron-electron interaction effects. On the other hand, excited state
ordering opposite to that in trans-polyacetylene is usually obtained by
chemical modification that increases the effective bond-alternation, which in
turn increases the optical gap. We develop a theory of electron correlation
effects in a model $\pi$-conjugated polymer that is obtained by replacing the
hydrogen atoms of trans-polyacetylene with transverse conjugated groups, and
show that the effective on-site correlation in this system is smaller than the
bare correlation in the unsubstituted system. An optical gap in the infrared as
well as excited state ordering conducive to light emission is thereby predicted
upon similar structural modifications.
