Provided are acedan derivatives having an extended π bond, a method for preparing the acedan derivatives, and a method for two-photon microscopy imaging of amyloid-beta plaque using the acedan derivatives; more particularly, to two-photon absorbing fluorescent compounds having a longer absorption wavelength and emission wavelength than acedan and acedan derivatives which are conventional two-photon absorbing fluorophores. The compounds provided may be usefully used for in vivo imaging studies by imaging cells or tissue using the compounds, and may also be usefully used for diagnosing Alzheimer's disease by imaging amyloid-beta plaque using the compounds.