Through-Space Charge Transfer via Spatial Distortion in Twisted PDI–Helicene Systems: A New Strategy for Nonlinear Optical Properties

The nonlinear optical (NLO) properties of twisted donor-acceptor (D-A) molecules composed of perylene diimide (PDI) and [6]helicene units were systematically investigated using density functional theory (DFT). The results indicate that the number of helicene units had a greater influence on the energy gap (E gap ) than their relative spatial arrangement, enabling efficient directional through-space charge transfer (TSCT) from [6]helicene to PDI. Molecule 1, bearing a single [6]helicene unit, exhibited the smallest E gap (2.531 eV), highlighting its potential as a narrow-bandgap organic semiconductor with pronounced CT characteristics. Notably, the first hyperpolarizability (β tot ) value was governed primarily by the consistency of the helicene rotation directions rather than by whether the helicene units were located on the same or opposite sides of the PDI core. Molecules 3 (1.8 × 10 −29 esu) and 5 (1.3 × 10 −29 esu) showed larger β tot values because their two helicene units rotated in the same direction, which induced synergistic chirality and dipole alignment, resulting in a stronger non-centrosymmetric charge distribution and enhanced β tot value. These findings emphasized that the rotation direction of the helicene units played a crucial role in regulating the TSCT and thereby controlling the NLO properties of twisted D-A molecules. These insights provide a new strategy for designing high-performance organic NLO materials.