Persistent room temperature blue phosphorescence from racemic crystals of 1,1-diphenylmethanol derivatives

Among the six alkyldiphenylmethanol derivatives assessed in this study, five showed persistent room temperature phosphorescence (pRTP), whereas the original diphenylmethanol (DPhHOH) did not. pRTP is a unique luminescence phenomenon that appears only in the crystalline state and is different from the luminescence produced by dispersed molecules, such as those in a glassy solvent matrix at 77 K or a polymeric matrix. The data also suggest that pRTP appears in conjunction with a specific crystal structure. The present work demonstrates that various crystal structures show pRTP regardless of the presence or absence of weak hydrogen bonds. However, nonradiative deactivation is suppressed by both intermolecular interactions and steric regulation in the crystal, such that the emission quantum yield is dependent on the crystal structure. In addition, although the molecular structure in the crystalline state necessary for the appearance of pRTP was not fully elucidated, intermolecular interactions between face-to-face and face-to-edge benzene rings appear to play a major role. Density functional theory calculations indicate that intersystem crossing is promoted in the crystalline state because the number of paths that allow this intersystem crossing is greater in a structure having weak interactions between molecules as compared to a dispersed state. DPhHOH did not show pRTP and is considered to have a structure based on the column-like stacking of multiple molecules. In contrast, the crystals that exhibited pRTP had pseudo racemic conformation in their crystals even though each molecule was achiral. In these racemic crystals, the interactions between pairs of molecules were important to the generation of pRTP.