Nonlinear optical spectroscopy of excited states in disubstituted polyacetylene

We used a variety of nonlinear optical (NLO) spectroscopies for studying the excited states order and primary photoexcitations in disubstituted polyacetylene (DPA). The NLO spectroscopies include ultrafast pump-probe photomodulation in a broad spectral range of 0.2-2.5 eV, two-photon absorption in the range of 3.2-4.2 eV, and electroabsorption (EA) in the range of 2.3-4.1 eV. For completeness we also measured the linear-absorption and photoluminescence spectra. We found that the primary photoexcitations in DPA are singlet excitons with three photoinduced absorption (PA) bands, PA0 at 0.6 eV, PA1 at 1 eV, and PA2 at 1.6 eV, respectively, and a stimulated emission band at ~2.5 eV. This is in contrast to other rr-conjugated polymers where the singlet excitons are characterized by two PA bands and a stimulated emission (SE) band. We conclude that in addition to the four essential excited states known to determine the NLO spectra of rr-conjugated polymers, namely, 1Ag, 1Bu, mAg, and nBu; in DPA there is another important complex of states at ~3.3 eV that contains almost degenerate states with odd and even parity symmetries (dubbed here transverse excited state or TES) that is probably reminiscent of the 2Ag state in unsubstituted trans-polyacetylene, which lies in between the 1Bu and mAg of which dipole moment has a strong component perpendicular to the chain direction. In support for this model we found that the depolarization ratio of the EA spectrum drastically changes around the TES from a value of ~2 for һω<3 eV to ~0.5 for һω>3.3 eV.