Synthesis and efficient three-photon excited green fluorescence of pyridine–triphenylamine conjugated dyes

• We synthesize pyridine-end-capped triphenylamine chromophores (Py2-TPA)n.
• These chromophores exhibit efficient three-photon excited green fluorescence.
• Increasing the conjugation push-pull units will increase 3PA cross-sections.
• Increasing extent of symmetrical charge transfer will increase 3PA cross-sections.
• Large aromatic chromophores (Py2-TPA)n provide a new opportunity for 3PA-materials.

Pyridine-end-capped triphenylamine derivatives (Py2-TPA)n (n = 1,2,4) were synthesized using palladium-catalyzed Heck reaction and characterized by 1H NMR, 13C NMR, and HRMS-MALDI-TOF. Three-photon absorption and three-photon excited fluorescence properties of chromophores (Py2-TPA)n were studied using a femtosecond Ti:Sapphire laser system. These chromophores exhibit efficient three-photon excited green fluorescence peaking at 540–590 nm. The three-photon absorption cross-section of σ3 = 6.99 × 10−79 cm6 s2 photon−2 in the femtosecond regime has been obtained from (Py2-TPA)4. The ratio of 3PA cross-sections in CH2Cl2 solution σ3 (Py2-TPA)4:σ3 (Py2-TPA)2:σ3 (Py2-TPA)1 is 6.0:4.2:1. The ratio of conjugated π-electron numbers in (Py2-TPA)n is 4.4(n = 4):2.6(n = 2):1(n = 1). The increasing the push–pull units of (Py2-TPA)n and π-electron conjugation length or increasing the extent of charge transfer from the ends to the middle aryl core results in an increase of the 3PA cross-section. The results indicate that large aromatic conjugated chromophores (Py2-TPA)n containing pyridine–triphenylamine D–A pairs and a conjugated aryl core provide a new opportunity for 3PA-materials and three-photon fluorophores for practical application.