Diphenylaminofluorene-based two-photon-absorbing chromophores with various π-electron acceptors

A new series of linear, asymmetrical diphenylaminofluorene-based chromophores (AFX) with various strong π-electron acceptors were synthesized and evaluated for two-photon absorptivity. These chromophores were studied to determine a suitable replacement for 2-(4-vinyl)pyridine, the π acceptor for our previously reported AFX series, which contains a photochemically and thermo-oxidatively unstable olefinic unit. In addition to the benzoyl group (AF-370), these π-electron acceptors include 2-benzothiazolyl (AF-240), 2-benzoxazolyl (AF-390), 2-(N-phenyl)benzimidazolyl (AF-386), and 2-(3,4-diphenyl)imidazolyl (AF-385) moieties (five-membered heterocycles) and the 2-quinoxalinyl (AF-260) group (six-membered heterocycle). From nanosecond nonlinear transmission measurements, these new chromophores have effective two-photon cross sections (σ₂′) at 800 nm, spanning from 3.87 × 10^-48 cm⁴ s/(photon molecule) for AF-385 to 97.46 × 10^-48 cm⁴ s/(photon molecule) for AF-240. Two of them, 2-benzothiazolyl-end-capped AF-240 and benzoyl-containing AF-370 [σ₂′ = 84.32 × 10^-48 cm⁴ s/(photon molecule)] stand out as having relatively good, albeit lower, values of two-photon cross sections, as compared to that of previously reported N,N-diphenyl7-[2-(4-pyridinyl)ethenyl]-9,9-didecyl-fluorene-2-amine, AF-50 [σ₂′ = 115.6 × 10^-48 cm⁴ s/(photon molecule)]. However, we observed that AF-240 was more photochemically robust than AF-50 when their THF solutions were subjected to repetitive and prolonged exposure to nanosecond laser radiation. Finally, on the basis of our nanosecond TPA cross-section data (σ₂′/MW values), the general trend for π-electron accepting ability, i.e., ability to accept charge transferred from diphenylamine appears to be as follows: 2-(4-vinyl)pyridine > 2-benzothiazolyl > benzoyl > 2-(N-phenylbenzimidazolyl > 2-quinoxalinyl > 2-benzoxazolyl > 2-(4,5-diphenyl)imidazoyl.