How do structural factors determine the linear and non-linear optical properties of fluorene-containing quadrupolar fluorophores? A theoretical answer

Abstract

A large series of push–push and pull–pull quadrupolar fluorophore derivatives with conjugated rods made from arylene–vinylene (PV) or arylene–ethynylene (PE) building blocks, bearing different electron-releasing or electron-withdrawing end-groups, were theoretically investigated using DFT and TD-DFT computations. These compounds, which exhibit good transparency in the visible region, are very promising for various applications, especially for optical limitation. Their absorption and photoluminescence as well as their two-photon absorption (TPA) properties in the near infrared (NIR) region were systematically investigated in order to derive structure–property relationships. The results indicate that (i) for all the studied compounds, the lowest excited state reached by the TPA is the S₂ state, corresponding to the HOMO−1 to LUMO electronic transition, (ii) push–push molecules are found to be more efficient than pull–pull molecules, (iii) an increase of the strength of the donor terminal group (OPh, OMe, NH₂, NPh₂, and NBu₂) enhances the TPA cross-section, and (iv) replacement of PV by PE always leads to an increase of TPA cross-sections in the NIR region.