PHOTOEXCITED SOLITONS AND POLARONS IN PERNIGRANILINE-BASE POLYMERS

We present results of absorption, near-steady-state and long-time photoinduced absorption, and light-induced electron-spin resonance (LESR) studies for the pernigraniline-base polymer (PNB), which has a Peierls ground state with multiple order parameters. Two kinds of defects have been observed and differentiated by photoinduced-absorption spectroscopies in different time domains, namely polarons and solitons. Direct absorption of PNB shows the Peierls energy gap at 2.3 eV and pi-pi* transition energies at 3.8 and 4.3 eV. Pumping at 2.41 eV (into the Peierls gap), the near-steady-state photoinduced absorption spectrum at room temperature shows three induced absorption peaks at 1.0 eV (LE), 1.5 eV (ME), and 3.0 eV (HE); at a temperature of 10 K, the LE peak becomes two peaks at 1.0 eV (LE1) and 1.3 eV (LE2). With pump photon energy of 3.8 eV (into the pi-pi* gap) and at 10 K, features similar to those observed by pumping into the Peierls gap were found, except the intensity of each of the features increased by a factor of 4. The LE features are short lived with a lifetime on the millisecond time scale and have associated short-lived infrared vibrations. The ME peak is very long lived; in addition, there are very long-lived, relatively weak infrared vibrations. The lifetime of the ME peak is measured to be greater than 24 h for temperatures below 200 K. The defect masses for long-lived and short-lived defects are estimated to be approximately 300m(e) and approximately (5-10)m(e), respectively, using the amplitude-mode formalism. Light-induced ESR studies show the long-lived 1.5 eV peak has spin while the short-lived defect has no spin. We suggest that the LE peaks originate from charged soliton-antisoliton pairs (dominated primarily by the bond-length order parameter u), while the ME peak originates from massive ring-torsional polarons (with dominant ring-torsion-angle order parameters delta and PSI0).