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A newly synthesized 1,3,4-oxadiazole derivatives have been studied using a femtosecond Ti:sapphire laser system.The series molecules present strong three-photon absorption and frequency upconversion fluorescence at wavelengths from 1205nm to 1575nm.Furthermore,there is no proportional relationship between three-photon absorption cross sections and the chemical structure transformation from monomer,dimer to trimer.Effective charge-transfer distance by π-conjugation bonds may be the contributing factor.In the experimental design,the far-field intensity distribution of femtosecond laeer beam has been taken into account.