Femtosecond excited-state dynamics of a conjugated ladder polymer.

Aniong the known classes of nonlinear optical materials, z-conjugated polymers are very attractive because of their large, third-order optical susceptibilities and ultrafast response times.lP5 The large optical nonlinearity of conjugated polymers has long been recognized as arising from the z-electron delocalization along the polynler chains2 Recently, aromatic heterocyclic ladder-type polymers, such as poly(benzimidazobenzophenanthroline) ladder (BBL) and semi-ladder (BBB), have received growing attention as a new class of nonlinear optical (NLO) polymers3-5 because of their desirable physical properties and their ready processibility into good opticalquality thin films6 For example, BBL has an excellent thermal stability up to 700°C in a nitrogen atmosphere (-650°C in air) and has good mechanical properties in the form of films or fiber^;^ both of these features suggest that the optical damage threshold may be very high. The rigid, planar, "double stranded," and quasi-two-dimensional structure of BBL (inset of Fig. 60.22) suggests maximum z-electron delocalization, leading to a large, third-order optical susceptibility X(3) as found in picosecond degenerate four-wave mixing (DFWM)~ and picosecond third-harmonic-generation (THG) experim e n t ~ . ~ However, the temporal resolution of the nonlinear optical response of BBL was limited to 30 ps by theexperimental equipment in both of these prior s t u d i e ~ . ~ . ~