Intersystem Crossing Mechanisms in the Room Temperature Phosphorescence of Crystalline Organic Compounds

Abstract

Reports on the room temperature phosphorescence of metalfree organic crystals have been surging in the past few years. Together with interests in the rare phenomenon, these compounds have attracted attention for such potential applications as bio-imaging probes, oxygen sensors, and organic lightemitting diodes. For common organic compounds, phosphorescence is the emission from a triplet excited state, which is usually produced from a singlet excited state through intersystem crossing, a forbidden spin-flip of an electron. The mechanism of the forbidden process is the key to understanding such rare phenomenon and designing new phosphorescence materials. In this account, we make commentaries on the main intersystem crossing mechanisms proposed to date of the room temperature phosphorescence of heavy-atom-free, crystalline organic compounds, focusing on our own findings.

Enhanced Room‐Temperature Phosphorescence through Intermolecular Halogen/Hydrogen Bonding

“…For metal-free organic materials,r ational molecular design strategies are alwaysn eeded to ensure a fast ISC rate (k ISC )t oc ompete with the rate of fluorescence decay (k F )a nd the rate of internal conversion (k IC )i nt he depopulating processes of S 1 . [23] The quantumy ield of ISC, ; ISC ,i s defined by Equation (1):…”

Engendering persistent organic room temperature phosphorescence by trace ingredient incorporation

“…Crystal packing ( 22 ), polymerization ( 23 ), and host-guest interaction ( 24 – 26 ) are analyzed to suppress the molecular vibration and oxygen quenching, which competed with the radiative transition. However, vibration restraint and avoiding quenching were not the sole factors for RTP, because not all materials could emit RTP in crystal state in vacuum ( 27 ). Other mechanisms for enhancing ISC were proposed by increasing heavy-atom effect, reducing the energy gap between singlet and triplet (Δ E ST ), and being subjected to the El-Sayed rule ( 28 , 29 ).…”

Efficient metal-free organic room temperature phosphors

“…A CT state can be introduced either by bridging donor–acceptor (D–A) groups or through the assistance of intermolecular interactions between donor and acceptor molecules. Such examples for enhanced RTP through small Δ E ST driven by the CT state 38 and through aggregation controlled management of Δ E ST (ref. 40 ) are available in the literature.…”