Vertical substitution strategy to enable cooperation between spin-orbit coupling and transition dipoles for organic phosphorescence.

The emission from organic molecules can be broadly classified as fluorescence or phosphorescence, and it is used in a wide range of applications from optoelectronics to bioimaging. The common method to enhance the fluorescence from organic chromophores is to introduce horizontal substituents into long conjugated structures. However, this concept does not allow the phosphorescence efficiency in the visible range to be increased to the fluorescence efficiency. Here we introduce a molecular design of conjugated molecules that effectively cooperate spin-orbit and transition dipoles. Increasing the through-space substitution of main-group elements perpendicular to the conjugated plane produces a cooperative effect of the spin-orbit and large transition dipoles of the conjugated plane to enhance the organic phosphorescence. This molecular design, which is different from the fluorescence, will increase the efficiency of the organic phosphorescence to the same level as the fluorescence at all visible wavelengths.