Ultrafast Spectroscopy Reveals Frenkel-CT Mixed Excitonic States in Copper Phthalocyanine

In organic semiconducting systems, intermolecular charge transport and energy diffusion take place along the $\pi $-$\pi $ stacking direction, which is beneficial for opto-electronic devices. Therefore it is essential to study electronic state structure in the $\pi $-$\pi $ stacking direction in organic solids.~ We studied a model quasi-one-dimensional molecular crystal copper phthalocyanine, which has strong intermolecular coupling along the $\pi $-$\pi $ stack. In this work, we used polarization resolved transient absorption spectroscopy and identified the coupling~of~low-lying singlet Frenkel (intramolecular) excitons with CT (intermolecular) excitons. Our study shows an evolution between localized and delocalized excitations which can be utilized to tune charge transport properties in molecular crystals. These studies provide fundamental understanding~of~electronic state structures, which will be essential for tailoring electronic properties~of~desired applications.