The Importance of Solid-state Molecular Motion to Room Temperature Phosphorescence

Molecular motion is often considered detrimental to luminescence because it favors nonradiative decay. However, nothing is absolute, and molecular motion can also do useful work if utilized properly. For example, photothermal therapy makes use of the heat generated in light irradiation for cancer treatment. To further explore the merits of molecular motion, ortho-substituted benzoic acids were used as model compounds to evaluate the importance of molecular motion to luminescence in the solid state. It is verified that the twisting of the carboxylic acid group can activate spin vibronic coupling to facilitate intersystem crossing to result in more efficient room temperature phosphorescence (RTP). A five-state model is established to understand the ISC process and an effective pre-twisted molecular design strategy is put forward for the development of efficient RTP materials.