Dimensional Control of Excitonic Interactions in Exfoliated 2D Molecular Crystals

Excited states in molecular crystals depend on intermolecular coupling, charge-transfer interactions, dielectric screening and molecular packing. How reduced dimensionality modifies these interactions when molecular packing structure is preserved remains unclear. Here we demonstrate that tetracene and three other molecular crystals can be mechanically exfoliated into mono-, few- or multilayer flakes, exploiting the top-down approach established for inorganic layered materials. Electron diffraction confirms that exfoliated tetracene retains long-range crystalline order and nearly bulk-like in-plane lattice parameters. This structural fidelity enables systematic thickness-dependent spectroscopy with limited changes in molecular packing. With decreasing thickness, the Davydov splitting diminishes, whereas the Stokes shift increases to nearly three times its bulk-like value, indicating enhanced excited-state relaxation. The photoluminescence 0-0/0-1 intensity ratio also rises, suggesting greater exciton delocalization. Compared with exfoliated tetracene, assembly-grown crystals exhibit lattice contraction and larger Davydov splitting. Analysis within the Frenkel-charge-transfer mixing framework qualitatively accounts for the Davydov-state energy shifts and splitting in terms of molecular packing, intermolecular coupling and dielectric screening. Our results show that intermolecular interactions, excited-state relaxation and exciton coherence can be systematically tuned by varying layer number, demonstrating the feasibility and utility of extending dimensional control of inorganic 2D materials to molecular crystals.