Ultrafast Transient Spectroscopy of Trans-Polyacetylene in the Midinfrared Spectral Range

Trans-polyacetylene [$t\text{−}{(\mathrm{CH})}_x$] possesses twofold ground state degeneracy. Using the Su-Schrieffer-Heeger Hamiltonian, scientists predicted charged solitons to be the primary photoexcitations in $t\text{−}(\mathrm{CH}{)}_x$; 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 $t\text{−}{(\mathrm{CH})}_x$ thin films. We show that odd-parity singlet excitons are the primary photoexcitations in $t\text{−}{(\mathrm{CH})}_x$, similar to many other nondegenerate $\pi$-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 $\sim 1.5\mathrm{eV}$ having similar polarization memory. The primary excitons undergo internal conversion within $\sim 100\mathrm{fs}$ to an even-parity (dark) singlet exciton with a PA band at $\sim 1.4\mathrm{eV}$. 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.

Figure 1

(a) The absorption spectrum of $t\text{−}{(\mathrm{CH})}_x$ thin film. (Inset) The parallel ($\parallel$) and perpendicular ($\perp$) components of the polarized PL emission spectrum with respect to the polarization of the pump excitation. O.D. means optical density. (b) cw photomodulation spectra of $t\text{−}{(\mathrm{CH})}_x$ at 80 and 200 K, respectively. The LE PA band was identified as being due to $S^{\pm }$ [13, 19], whereas the HE PA band was ascribed to $S^0$ [13]. (c) Action spectrum of the LE PA band in the steady state PM spectrum. (d) Transient PM spectra of $t\text{−}{(\mathrm{CH})}_x$ film pumped at 3.1 eV (blue squares and line) and 1.55 eV (red circles and line) compared with the steady state PM spectrum (black line) measured at 80 K. The lines through the data points are guides for the eye.

Figure 2

Transient PM spectra and decay dynamics of $t\text{−}{(\mathrm{CH})}_x$ film in the mid-IR spectral range excited at 1.55 eV. (a) The transient PM spectra at $t=0$ (red circles) and $t=1\mathrm{ps}$ (blue squares). The lines through the data points are to guides for the eye. Various bands are assigned (defined in the text). TPA is pump-probe two-photon absorption. (b) Schematic of the energy levels and optical transitions of singlet excitons. $1B_u$, $n{B}_u$, $1A_g$, $2A_g$, $m{A}_g$, and $k{A}_g$ are odd- and even-parity exciton states in the neutral manifold. IC is the internal conversion from the $1B_u$ state to the $2A_g$ state. (c) The transient rise and decay dynamics of the main PA bands assigned in (a). (d) Transient polarized response of ${\mathrm{PA}}_2$; both the $\mathrm{\Delta }{T}_{\parallel }$ and $\mathrm{\Delta }{T}_{\perp }$ components are shown.

Figure 3

Transient PM spectra and decay dynamics of $t\text{−}{(\mathrm{CH})}_x$ film in the mid-IR spectral range excited at 3.1 eV. (a) The transient PM spectra at $t=0$ (red circles) and $t=10\mathrm{ps}$ (blue squares); the lines through the data points are guides for the eye. Various PA bands are assigned. (b) Schematic of the energy levels and optical transitions of charge polaron excitation. HOMO (LUMO) is the highest (lowest) occupied (unoccupied) molecular orbital in the charge manifold. (c) The transient decay dynamics of the main PA bands assigned in (a). (d) Transient decay dynamics of the $P_1$ and $P_2$ bands plotted in log-log scale.