An Organic Laser Dye having a Small Singlet-Triplet Energy Gap Makes the Selection of a Host Material Easier

Quenching of singlets by long-lived triplets is a serious issue for lasing from organic laser dyes, especially under long pulse excitation. As a strategy to scavenge or manage unnecessary triplets, an organic laser dye is dispersed into a host material having high singlet and low triplet energy levels [a large singlet-triplet energy gap (ΔE_ST)]. However, finding such a host material having a triplet scavenging capability is limited. In this study, an organic laser dye, 2,6-dicyano-1,1-diphenyl-λ⁵σ⁴-phosphinine (DCNP), having a small ΔE_ST of ≈0.44 eV is synthesized, and thus 4-4′-bis[(N-carbazole)styryl] biphenyl (BSBCz) can be employed as a triplet scavenging host, i.e., the triplets formed on DCNP are easily transferred to BSBCz. A 1 wt%-DCNP-doped BSBCz film is formed on a mixed-order distributed feedback grating, showing lasing with a low threshold value of ≈0.86 µJ cm⁻² and a full-width-at-half-maximum value of ≈0.5 nm. Because of the suppressed singlet-triplet annihilation, DCNP-based laser devices operating under a continuous-wave regime, with a low threshold of 72 W cm⁻² and a long laser half-lifetime of ≈3 min, are demonstrated. These results indicate a possibility of the wider selection of host materials, easing a material design strategy of fabricating high-performance laser devices in future.