Toward Highly Active Two-Photon Absorbing Liquids. Synthesis and Characterization of 1,3,5-Triazine-Based Octupolar Molecules

Three novel two-photon absorbing (TPA) chromophores with 1,3,5-triazine as the π-electron deficient core, dialkylfluorene as aromatic bridges, and diphenylamino groups as the electron-donating end-groups were prepared. Designated as AF-450 (2,4,6-tris[7-(diphenylamino)-9,9-didecylfluoren-2-yl]-1,3,5-triazine), AF-455 (2,4,6-tris[9,9-bis(3,7-dimethyloctyl)-7-(diphenylamino)-fluoren-2yl]-1,3, 5-triazine), and AF-457 (2,4,6-tris [(7-(diphenylamino)-9,9-diprop-2-enylfluoren-2-yl]-1,3,5-triazine), their overall molecular structure and local symmetry (D_3h) are similar to those of previously reported three-armed AF-350 (N,N,N-tris[4-{7-(2-benzothiazolyl)-9,9-diethylfluoren-2-yl}phenyl]amine) and AF-380 (N,N,N,-tris[7-(2-benzothiazolyl)-9,9-diethylfiuoren-2-yl]amine). Among the family of AFX chromophores previously reported by us, AF-450 possesses one of the largest effective TPA cross-sections (σ₂′ = 39 500 × 10^-50 cm⁴-sec/photon-molecule, or 39 500 GM) as determined by nonlinear transmission method in the nanosecond regime at 800 nm. In contrast, AF-455, a mixture of stereoisomers with the same chemical formula as AF-450, is a glassy material that becomes fluid (molasses-like) upon heating at 70-80 °C and has noticeably smaller effective σ ₂′ value (33 300 GM). AF-457 (σ₂′ = 27 800 GM) with six allyl side groups was prepared as a precursor toward the synthesis of a TPA liquid. The intrinsic TPA cross-sections of these chromophores were also determined as a function of excitation wavelengths via a femtosecond white-light continuum generation and direct degenerate-TPA measurement technique. At the TPA peaks ∼779 nm, their σ ₂′ values are 216, 214, and 199 GM (±15%) for AF-450, AF-455, and AF-457, in that order. They are in the same trend as the nanosecond values, albeit two orders of magnitude lower.