Pure Organic Persistent Room‐Temperature Phosphorescence at both Crystalline and Amorphous States

Abstract

Persistent room-temperature phosphorescence (p-RTP) of pure organic materials is attracting increasing attention. The design of efficient phosphors and understanding the origin of p-RTP, however, remain challenging. Herein, to gain further insights into pure organic p-RTP, we prepared a group of carbazole (CZ) and methyl benzoate (MBA) conjugates with a methyl ester unit at the para (4-MBACZ), meta (3-MBACZ), and ortho (2-MBACZ) sites. These isomers merely produce prompt fluorescence in solutions, but generate blue prompt/delayed fluorescence (DF) and orange p-RTP with lifetimes up to 865.2 ms in the crystalline state. Lifetimes of p-RTP are in the order of 2-MBACZ>3-MBACZ>4-MBACZ, which might be mainly ascribed to the combined effect of packing density, intermolecular interactions and steric hindrance. Meanwhile, upon mechanical grinding, while the emission color and profile of the luminogens do not significantly change, no (4-MBACZ) or shortened (3-MBACZ, 2-MBACZ) p-RTP is observed, accompanying the change from crystalline to amorphous states. Such p-RTP at amorphous states without external hosts is rarely reported, which demonstrates important implications for the molecular design and mechanism understanding towards p-RTP. Furthermore, their p-RTP attribute and different emission colors before and after ceasing the UV irradiation endow them promising applications in encryption and anticounterfeiting fields.

Crystal Engineering of Room Temperature Phosphorescence in Organic Solids

“…Bryce and co‐workers proposed that the ISC is enhanced by intermolecular electronic coupling of the “π” (carbazole) and “n” (carbonyl or sulfonyl) moieties, and demonstrated that pure (fluorescence‐free) ORTP (PLQY ≈5 %) can be achieved in such organic solids when substituted with bromine . Several groups have further studied the effect of solid state structure on phosphorescent properties through permutation of substituents in the molecule . However, predictability of the induced changes and the generality of the deduced structure‐properties relationships are limited by the complexity of the crystal packing of these low‐symmetry molecules.…”

Manipulating the Ultralong Organic Phosphorescence of Small Molecular Crystals

“…According to El‐Sayed's rule (Figure b), ISC is spin‐allowed from 1 (n, π*) singlet to 3 (π, π*) triplet excited states and from 1 (π, π*) singlet to 3 (n, π*) triplet excited states, whereas ISC is spin‐forbidden from 1 (n, π*) singlet to 3 (n, π*) triplet and from 1 (π, π*) singlet to 3 (π, π*) triplet states, on account of minimal orbital overlapping, which leads to an inefficient spin‐orbit coupling. As for deactivation of T 1 to S 0 , 3 (n, π*) to π 2 is spin‐permitted and fast, whereas 3 (π, π*) to π 2 is spin‐prohibitive and relatively slow . So to achieve metal‐free organic phosphorescence with ultralong lifetimes and high quantum yields, these key factors are indispensable: (1) high Φ ISC values from S 1 to T n , (2) low k r and k nr from T 1 to S 0 , (3) a rigid environment to obtain low k q…”

Carbazole isomers induce ultralong organic phosphorescence

“…During the past 5 years, Cz derivatives have attracted much research interest, and many of them have directly led to the recent breakthroughs, such as highly efficient delayed fluorescence emitters 2,3 , efficient organic luminescent radicals 4 and organic semiconductor lasers 5 . In particular, commercial Cz derivatives are the current focus of single-component organic ultralong phosphorescence studies 1,[6][7][8][9][10][11][12][13][14][16][17][18] . However, fundamental inconsistencies emerged when the same compounds were repeatedly reported by different research groups [7][8][9][10][11][12]15 .…”

“…In particular, commercial Cz derivatives are the current focus of single-component organic ultralong phosphorescence studies 1,[6][7][8][9][10][11][12][13][14][16][17][18] . However, fundamental inconsistencies emerged when the same compounds were repeatedly reported by different research groups [7][8][9][10][11][12]15 . We therefore examined the commercial Cz from Tokyo Chemical Industry (TCI), J&K, Sigma-Aldrich (Sigma) and Aladdin.…”