In-Plane and Out-of-Plane Excitonic Coupling in 2D Molecular Crystals

Understanding the nature of molecular excitons in low-dimensional molecular solids is of paramount importance in fundamental photophysics and various applications such as energy harvesting, switching electronics and display devices. Despite this, the spatial evolution of molecular excitons and their transition dipoles have not been captured in the precision of molecular length scales. Here we show in-plane and out-of-plane excitonic evolution in quasi-layered two-dimensional (2D) PTCDA crystals assembly-grown on hexagonal BN crystals. Complete lattice constants with orientations of two herringbone-configured basis molecules were determined with polarization-resolved spectroscopy and electron diffraction methods. In the truly 2D limit of single layers, two Frenkel emissions Davydov-split by Kasha-type intralayer coupling exhibited energy inversion with decreasing temperature, which enhances excitonic coherence. As increasing thickness, the transition dipole moments of newly emerging charge transfer excitons are reoriented because of mixing with the Frenkel states. The current spatial anatomy of 2D molecular excitons will inspire deeper understanding and thus ground-breaking applications of low-dimensional molecular systems.