Solvent effect on two-photon absorption (TPA) of three novel dyes with large TPA cross-section and red emission

Three novel organic dyes containing 2,6-dicarbonyl-pyridine as the central electron acceptor with different end-caped amino groups as donors in the D-π-A-π-D arrangement are designed and synthesized. All three molecules exhibit large TPA cross-sections (δ > 1000 GM). The cooperative effects of donor strength, conjugation length and solvent polarity on the TPA activity are studied. The conjugated system plays an important role on the enhancement of δ in the highly polar solvent, while donor strength has more impact on TPA properties. The influence of the solvent polarity on TPA properties is studied by two-photon excited fluorescence (TPEF) measurement. The result shows a nonmonotonic relationship between the TPA cross-section and the solvent, and the maximum of δ is found in solvents with moderate polarity. The δ of molecules with stronger donor reaches the maximum in more polar solvents. The highest δ is found to be 5250 GM in CHCl3. All compounds exhibit large Stokes shifts due to the strong intramolecular charge transfer (ICT) effect.

► Large TPA cross-section was obtained on three small molecules.
► Strengthened acceptor consisting two strong electron-withdrawing groups.
► Nonmonotonic but regular TPA response towards the variation of solvent polarity.
► Red emission was observed in one of the three molecules.