The present invention relates to a complex comprising at least one dendrimer and at least one lanthanide, wherein the dendrimer comprises a unit of formula (I) below:in which:C1 is a valence group 4 of formula >;N—;CH2—;CH2—;N<;;A1, A2 and A3 are groups of formula —;(CH2)2—;C(O)—;NH—;(CH2)2—;; the unit of formula (I) being connected covalently to at least one antenna which absorbs at a wavelength ranging from 500 nm to 900 nm.