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 (σ2PA and σ3PA), 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 σ2PA, 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 σ3PA 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.