Ultrafast Relaxation in Quasi-One-Dimensional Organic Molecular Crystals

We present a comprehensive study of ultrafast relaxation properties of optical excitations in thin films of quasi-1D stacked organic materials PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI ($N,N^{\prime }$-dimethylperylene-3,4,9,10-dicarboximide) over five decades of time. Pump-probe experiments reveal excitonic intraband relaxation time constants of 65 fs for MePTCDI and 100 fs for PTCDA. The initial time-resolved luminescence anisotropy is consistent with the exciton model of Davydov-split states. The subsequent decay of the anisotropy can be explained with a thermally activated exciton hopping process. A full understanding of the pump-probe experiments calls for an explanation beyond the models presently available.

Figure 1

Top: linear absorption (dash-dotted line) and transient transmittance at 1 ps delay (thick solid line) of MePTCDI (a) and PTCDA (c) at room temperature. Dotted curves: transient luminescence (0–50 ps) at 10 K. Shaded area: probe beam spectrum. Bottom: fs-pump-probe differential transmittance for parallel polarization for MePTCDI (b) and PTCDA (d). Dashed lines: integrated cross-correlation curves.

Figure 2

Top: mutual orientation of two molecules in the unit cells of MePTCDI (a) [3] and PTCDA (b) [24]. Stacking direction is roughly perpendicular to the drawing pane. Superposition of the major molecular transition dipole moments ${\mathbf{P}}_{\mathrm{A}}$ and ${\mathbf{P}}_{\mathrm{B}}$. Bottom: overview of excitation and population, intraband relaxation, and the transitions allowed in indirect emission. In the example (c) ${\xi }^2=1/3$.

Figure 3

Polarization-dependent time-resolved photoluminescence and the corresponding luminescence anisotropies computed as defined by Eq. (1) for MePTCDI and PTCDA. $p$ $(s)$ denotes parallel (perpendicular) polarization. (a) and (c) are at 295 K; (b) and (d) are at 10 K.

Figure 4

Polarization-dependent ps-pump-probe measurements and the resulting anisotropies for MePTCDI [(a),(b)] and PTCDA [(c),(d)]. $p$ and $s$ denote parallel and perpendicular polarization of pump and probe beam, respectively.