Frequency‐Upconverted Stimulated Emission by Up to Six‐Photon Excitation from Highly Extended Spiro‐Fused Ladder‐Type Oligo(p‐phenylene)s

Frequency‐upconverted fluorescence and stimulated emission induced by multiphoton absorption (MPA) have attracted much interest. As compared with low‐order MPA processes, the construction of high‐order MPA processes is highly desirable and rather attractive, yet remains a formidable challenge due to its inherent low transition probability. We report the observation of the first experimental frequency‐upconverted fluorescence and stimulated emission by simultaneous six‐photon excitation in an organic molecular system. The well‐designed organic conjugated system based on cross‐shaped spiro‐fused ladder‐type oligo(p‐phenylene)s (SpL−z, z=1–3) manifests reasonably high MPA cross‐sections and brilliant luminescence emission simultaneously. The six‐photon absorption cross‐section of SpL‐3 with an extended π‐conjugation was evaluated as 8.67×10−169 cm12 s5 photon−5. Exceptionally efficient 2‐ to 6‐photon excited stimulated emission was achieved under near‐infrared laser excitation. Efficient frequency‐upconverted fluorescence and stimulated emission have been demonstrated in spiro‐fused ladder‐type oligo(p‐phenylene)s through multiphoton absorption with an absorbed photon number up to six. The results demonstrate an effective molecular design strategy for exploring highly efficient MPA fluorophores for multiphoton optical applications.