Temperature-Regulated Dual Phosphorescence and Mechanical Strain-Induced Luminescence Modulation in a Plastically Bendable and Twistable Organic Crystal

A unique, thermally interchangeable, and tunable dual phosphorescence from a single-component organic crystal of DBC (3,6-dibromo-9-ethyl-9 H -carbazole) is described here. Temperature-regulated relative intensities of a triple-channel radiative process, viz., singlet and dual triplets, enabled a near-white luminescence in a wide temperature range. Interestingly, the single crystals of DBC can also be plastically bent using external mechanical stress and allow the modulation of phosphorescence. The luminescence was completely singlet-mediated in the monomeric state, while the crystalline solid had three pathways of radiative processes. Strong n−π interactions mediated by non-covalent interactions significantly augment the initially forbidden n−π* transitions, which enabled a strong spin–orbit coupling to obtain large radiative triplet states in the solid state. Trapped higher energy triplets were the significant contributors to the emission at lower temperatures, whereas, at higher temperatures, the lower lying triplets dominated the process. A detailed mechanistic approach to unraveling the findings is summarized in the article.