Highly Efficient Multiphoton‐Pumped Frequency‐Upconversion Stimulated Blue Emission with Ultralow Threshold from Highly Extended Ladder‐Type Oligo( p ‐phenylene)s

A series of highly extended π‐conjugated ladder‐type oligo( p ‐phenylene)s containing up to 10 phenyl rings with ( L )‐Ph( n )‐NPh ( n= 7–10) or without diphenylamino endcaps ( L )‐Ph( n ) ( n= 7 and 8) were synthesized and investigated for their multiphoton absorption properties for frequency upconverted blue ASE/lasing. Extremely large two‐photon absorption (2PA) cross‐sections and highly efficient 2PA ASE/lasing with ultralow threshold were achieved. ( L )‐Ph(10)‐NPh exhibits the highest intrinsic 2PA cross‐section of 3643 GM for a blue emissive organic fluorophore reported so far. The record‐high 2PA pumped ASE/lasing efficiency of 2.06 % was obtained by un‐endcapped oligomer, ( L )‐Ph(8) rather than that with larger σ 2 , suggesting that a molecule with larger σ 2 is not guaranteed to exhibit higher η 2 . All of these oligomers exhibit exceptionally ultralow 2PA pumped ASE/lasing thresholds, among which the lowest 2PA pumped threshold of circa 0.26 μJ was achieved by ( L )‐Ph(10)‐NPh.