Role of Dimensionality on the Two‐Photon Absorption Response of Conjugated Molecules: The Case of Octupolar Compounds

A comparative study of the two-photon absorption (TPA) properties of octupolar compounds and their dipolar one-dimensional counterparts is presented on the basis of correlated quantum-chemical calculations. The roles of dimensionality and symmetry are first discussed on the basis of a simple exciton picture where the ground-state and excited-state wavefunctions of three-arm octupolar systems are built from a linear combination of the corresponding single-arm wavefunctions. This model predicts a factor of 3 increase in the TPA cross section in the limiting case of three independent charge-transfer pathways. When taking into account the full chemical structures of representative octupolar molecules, the results of the calculations indicate that a much larger enhancement associated with an increase in dimensionality and delocalization can be achieved when the core of the chromophore allows significant electronic coupling among the individual arms. These theoretical predictions are in agreement with the experimental determination of the TPA cross sections for crystal violet and the related compound, brilliant green, and suggest new strategies for the design of conjugated materials with large TPA cross sections.