Luminescence of doublet molecular systems

Abstract

Luminescent molecules are functional materials that are attractive for use in various fields such as in light-emitting devices and bio-imaging. These molecules generally possess a closed-shell singlet ground state and show fluorescence from the lowest singlet excited state or phosphorescence from the lowest triplet excited state. This review focuses on “open-shell” luminescent molecules with spin “doublet” ground states such as organic radicals, radical-ligated metal complexes, and metal complexes with magnetic metal ions as a rapidly growing class of molecular emitters. They exhibit unique luminescence properties that originate mainly from their emissions being based on the spin-allowed doublet–doublet transition. Their emission properties, responsiveness to external stimuli and environments, and photofunctional applications described here make doublet molecular systems promising solutions to problems that are intractable using conventional closed-shell molecular systems. These luminescent doublet molecular systems will provide a seminal perspective on molecular photochemistry.