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

A new series of linear, asymmetrical diphenylaminofluorene-based chromophores (AFX) with various strong pi -electron accepters were synthesized and evaluated for two-photon absorptivity. These chromophores were studied to determine a suitable replacement for 2-(4-vinyl)pyridine, the it 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 pi -electron accepters 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 (sigma (2)') at 800 nm, spanning from 3.87 x 10(-48) cm(4) s/(photon molecule) for AF-385 to 97.46 x 10(-48) Cm-4 s/(photon molecule) for AF-240. Two of them, 2-benzothiazolyl-end-capped AF-240 and benzoyl-containing AF-370 [sigma (2)' = 84.32 x 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-diphenyl-7-[2-(4-pyridinyl)ethenyl] -9,9-didecyl-fluorene-2-amine, AF-50 [sigma (2)' = 115.6 x 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 (sigma (2)'/MW values), the general trend for pi -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.