Quantitative Correlation Between Charge Transfer and Two Photon Absorption in Phenanthridine‐TCNB Cocrystal

ABSTRACT Charge transfer (CT) in organic donor‐acceptor (D‐A) systems is a spin‐allowed thermodynamic process. It creates new energetic landscapes and fundamentally decides charge photogeneration and transport, luminescent, nonlinear optical (NLO), and magnetic properties of solid‐state materials and devices. However, despite of efforts to quantify the general chemical structure‐molecular stacking‐performance relations in inorganic, perovskite, and organic materials, we currently still lack a clear picture on how this interaction affects the resulted electronic and photonic properties in D‐A systems. Herein, we design and grow a new cocrystal of donor phenanthridine (Phn) and acceptor 1,2,4,5‐Tetracyanobenzene (TCNB) and describe the relationship between molecular packing, charge transfer, photophysical, and two‐photon absorption (TPA) properties. The Phn‐TCNB cocrystals (PTCs) are investigated by diverse experimental analysis and quantum chemical computation. The lattice has a mixed stack of Phn and TCNB molecules with a D‐A intermolecular distance ∼3.4 Å along its 100 growth direction and is stable under a wide low temperature range. PTCs show a CT ground state with a theoretically estimated degree of 0.02 e, static dipole moment of 2.84 D, and a new excited state energetic diagram. For NLO response, PTCs exhibit TPA enabled emission at 473 nm under the excitation between 720 and 880 nm. Theoretical calculations on this molecular complex reveal its static first hyperpolarizability β vec of −406 a.u., and the two‐photon transitions in such D‐A system are understandable by the large dipole moment difference Δ μ ff between the corresponding excited state and the ground state. This work thus serves as a reference example that illustrates the relationship between molecular packing, charge transfer, and TPA properties in this D‐A system, offering insights for future advanced NLO materials development.