Abstract: A polymerization initiator molecule, excitable by two photons and capable of generating polymerization-initiating free radicals, includes two branches grafted onto a central phenyl nucleus. Each branch includes an oligomer of oligophenyleneethynylenyl type or oligo-2,5-dihalogenphenyleneethynylenyl type. A photopolymerizable composition, activatable by two-photon absorption, includes a radically polymerizable resin and a photochemically effective amount of a radical photoinitiator system. The photoinitiator system includes at least one initiator molecule as described above. Moreover, a method and an associated device for two-photon three-dimensional printing are disclosed. The method includes transforming a volume of a photopolymerizable composition including at least one initiator molecule. The transformation includes irradiating the volume of composition with an irradiation light source emitting an irradiation signal having a wavelength Lirr of between 1 and 1.5 times, and preferably between 1.1 and 1.