Aromatic Polyimides Containing Main-Chain Diphenylaminofluorene-Benzothiazole Motif: Fluorescence Quenching, Two-Photon Properties, and Exciplex Formation in a Solid State

[摘要]：A new bis(4-aminophenoxy) monomer containing a two-photon absorbing (2PA) and fluorescent diphenylaminodiethylfluorene-benzothiazole chromophore (AF240) was synthesized and used as a comonomer in preparing a series of heat-resistant, 2PA-active polyimides. Highly organo-soluble, these polymers easily formed optically clear, but nonfluorescent, films that contained covalently bound, AF240-like dye in concentrations up to similar to 1.0 M. For comparison purposes, a model compound (AF349) with phthalimido end-caps was also prepared. From the fluorescence data, the presence of phthalimido moieties in both the model compound and the polymers drastically quenched the fluorescence emission from the 2PA moieties of these materials in solution as well as in solid state. Their intrinsic (at 780 nm and with 160 fs pulses) and effective (at 800 nm and 8 ns pulses) two-photon properties in THF solutions (0.02 M), and film samples (40-65 mu m thick) were determined by a direct nonlinear transmission technique. Thus, their intrinsic (sigma((2))(fs)) and effective cross-section (sigma((2))(ns)) values in solution are 7.7-37 and 1070-6000 GM rer repeat unit, respectively. Surprisingly for the film samples, while the sigma((2))(fs) values agree with those determined in solutions, the sigma((2))(ns) values are 7-9 times larger, depending on the dye content. On the basis of the results of photophysical characterization of these polymers, we propose that on a nanosecond or slower time scale a stabilized/confined excited state complex is formed in an intrachain mode between an AF240-like moiety and a phthalimido unit to reasonably account for the fluorescence quenching and enhancement in the effective two-photon responses in film form.

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