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The three-photon absorption (3PA) properties of two thiophene-fluorene derivatives (abbreviated as MOTFTBr and ATFTBr) have been determined by using a Q-switched Nd:YAG laser pumped with 38ps pulses at 1064nm in DMF.The measured 3PA cross-sections are 152×10-78cm6s2 and 139×10-78cm6S2,respectively.The optimized structures were obtained by AM1 calculations and the results indicate that these two molecules show nonplanar structures,and attaching different donors has different effects on the molecular structure.The charge density distributions during the excitation were also systematically studied by using AM1 method.In addition,an obvious optical power limiting effect induced by 3PA has been demonstrated for both derivatives.