Electronic Absorption, Emission, and Two-Photon Absorption Properties of Some Extended 2,4,6-Triphenyl-1,3,5-Triazines

We report herein the linear optical properties of some extended 2,4,6-triphenyl-s-triazines of formula 2,4,6-[(1,4-C6H4)C≡C(4-C6H4X)]3-1,3,5-(C3H3N3) (3-X; X = NO2, CN, OMe, NMe2, NPh2) and related analogues 4 and 7-X (X = H, NPh2), before briefly discussing their two-photon absorption (2PA) cross-sections. Their 2PA performance is discussed in relation to 2PA values previously measured for closely related octupoles such as N,N′,N″-triphenylisocyanurates (1-X, 5, and 6-X) or 1,3,5-triphenylbenzenes (2-X). While s-triazines are usually much better two-photon absorbers in the near-IR range than these molecules, especially when functionalised by electron-releasing substituents at their periphery, they present a decreased transparency window in the visible range due to their red-shifted first 1PA peak, in particular when compared with corresponding isocyanurates analogues. In contrast, due to their significantly larger two-photon brilliancy, 2,4,6-triphenyl-s-triazines appear more promising than the latter for two-photon fluorescence bio-imaging purposes. Rationalisation of these unexpected outcomes is proposed based on DFT calculations.