Fig. 1. The schematic crystal structure of (a) PEPI and (b) PBPI.

Fig. 1. (a) Steady-state absorption and (b) PL emission spectra recorded on the thin-film samples of PEPI and PBPI at room temperature.

Fig. 2. The schematic crystal structure of (a) PEPI and (b) PBPI.

Fig. 2. Temperature-dependent PL emission spectra recorded on the thin-film samples of (a) PEPI and (b) PBPI in the temperature range from 297 K down to 77 K (at an interval of 10 K). (c) Representative retrieval of the excitonic X band from the bimodal profile (through Lorentzian decomposition) for PEPI at 77 K. (d) The plot of FWHM as a function of temperature for both PEPI and PBPI. The solid curves are the fits.

Fig. 3. Representative room-temperature fs-TA spectra (probe delays at 0.5, 5, 20, and 500 ps) recorded on the thin-film samples of (a) PEPI and (b) PBPI. Decay-associated spectra (via a global analysis) for (c) PEPI and (d) PBPI, with all the derived characteristic time constants being annotated therein.

Fig. 4. Temperature-dependent fs-TA spectra (probe delay at 0.5 ps) recorded on the thin-film samples of (a) PEPI and (b) PBPI in the temperature range from 275 K down to 77 K. Representative 77-K spectral profiles (probe delays at 160, 300, and 890 fs) for (c) PEPI and (d) PBPI, with all the excitonic resonances being annotated therein.

Fig. 5. (a) Typical fs-TA kinetic trace recorded on the thin-film sample of PEPI (probe at 528 nm, 77 K). (b) Oscillation dynamics of coherent phonons extracted from (a) via background subtraction. (c) Typical fs-TA kinetic trace recorded on the thin-film sample of PBPI (probe at 519 nm, 77 K). (d) Oscillation dynamics of coherent phonons extracted from (c) via background subtraction. The insets of (c) and (d) exhibit the corresponding low-frequency phonon modes derived via Fourier transformation.