Multiphoton Absorption in Branched Quadrupolar Diquinoxalines for Two-Photon Polymerization Design

We investigated the photophysical properties of three branched diquinoxaline (DQ) derivatives and their potential application as photoinitiators (PIs) for two-photon polymerization (2PP). These compounds exhibit a quasi-centrosymmetric quadrupolar configuration, functionalized with different electron-donating (ED) moieties. We performed detailed linear spectroscopic characterization, as well as the determination of two- and three-photon absorption cross sections (σ_2PA and σ_3PA ), using the spectrally resolved multiphoton-excited fluorescence technique. We complemented the study by performing quantum chemical calculations at the (TD-)-DFT level to obtain relevant information about the electronic structure of the compounds and to model their spectroscopic data. The results showed that the compounds exhibit broad one-photon absorption in the UV-vis region and fluorescence emission at ca. 543 nm. Due to their quadrupolar nature, all molecules exhibit significant intramolecular charge transfer. Despite their essentially quadrupolar nature, we observed a symmetry-breaking effect in the relaxed excited state, conferring a dipolar character to the molecules. We observed significant σ_2PA , exhibiting the values up to 230 GM, which was attributed to the enhanced structural planarity, facilitating electron delocalization across all ED branches and resulting in a more efficient additive effect on 2PA response. Additionally, we noted a σ_3PA of approximately 0.02 × 10^-78 cm⁶ s² photon^-2 at 1100 nm. Finally, we confirmed the ability of the DQ derivatives to act as two-photon PIs, as demonstrated by the fabrication of 3D microstructures with inherent emissive properties.