Highly Efficient Room‐Temperature Phosphorescence Promoted via Intramolecular‐Space Heavy‐Atom Effect

Abstract Purely organic room‐temperature phosphorescence (RTP) materials have attracted increasing attention due to their unique photophysical properties and widespread optoelectrical applications, but the pursuit of high quantum yield is still a continual struggle for RTP emission under ambient conditions. Here, a series of novel RTP molecules (26CIM, 246CIM, 24CIM, and 25CIM) are developed on the basis of indole luminophore, in which a carbonyl group bridges indole and chloro‐substituted phenyl group. The structural isomerism is systematically regulated toward enhancing the intramolecular‐space heavy‐atom effect, thus promoting the spin–orbit coupling and intersystem crossing for high RTP efficiency. While rationally modulating the intramolecular‐space heavy‐atom effect, the phosphorescence efficiency is dramatically increased by 16‐fold from 2.9% (24CIM) to 48.9% (26CIM). Basically, the fully occupied chlorine atoms at the positions 2 and 6 can effectively favor the stronger intramolecular H^…Cl effect, and the tight lock coupling with anti‐parallel stacking in 26CIM further boosts RTP emission synergistically. The experimental findings along with deeper theoretical insights elucidate the structure–performance relationship clearly, and further suggest a general strategy for rationally constructing high‐efficiency RTP materials.