Trans-polyacetylene [t−(CH)x ] possesses twofold ground state degeneracy. Using the Su-Schrieffer-Heeger Hamiltonian, scientists predicted charged solitons to be the primary photoexcitations in t−(CH)x ; this prediction, however, has led to sharp debate. To resolve this saga, we use subpicosecond transient photomodulation spectroscopy in the mid-IR spectral range (0.1–1.5 eV) in neat t−(CH)x thin films. We show that odd-parity singlet excitons are the primary photoexcitations in t−(CH)x , similar to many other nondegenerate π -conjugated polymers. The exciton transitions are characterized by two photoinduced absorption (PA) bands at 0.38 and 0.6 eV, and an associated photoluminescence band at ∼1.5  eV having similar polarization memory. The primary excitons undergo internal conversion within ∼100  fs to an even-parity (dark) singlet exciton with a PA band at ∼1.4  eV . We also find ultrafast photogeneration of charge polarons when pumping deep into the polymer continuum band, which are characterized by two other PA bands in the mid-IR and associated photoinduced IR vibrational modes.

Authors : C. X. Sheng, K. H. Kim, M. Tong, C. Yang, H. Kang, Y. W. Park, and Z. V. Vardeny
Journal : Phys. Rev. Lett. 124, 017401 - Published 8 January 2020

Link : https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.017401