Innovative Organic Electroluminescent Materials with a Doublet Ground State: A Theoretical Investigation

Highly efficient NIR-Ⅱ window photoluminescence up to 1000 nm using heteroatomic fused-ring radicals

Neutral radicals have the potential to construct pure organic light-emitting diodes (OLEDs) with internal quantum efficiencies reaching 100%. However, neutral radical luminescent materials with emission wavelengths in the second near-infrared (NIR-II) window are rare. Herein, a serial of neutral donor-bridge-acceptor (D-π-A) type radical derivatives are investigated. The dominant elements influencing the luminescent properties of neutral radicals, such as chemical stability, excited state characteristics, radiative decay rate (kr) and internal conversion rate (kIC) constants are taken into consideration. Theoretical calculations reveal that introducing heteroatomic fused-rings into neutral radicals can modulate the chemical stability and result in a red shift of the emission wavelength spectrum. In the presence of charge transfer characteristics, by increasing the effective overlap between the hole and electron wavefunctions, the kr constants of the neutral D-π-A type radicals increase. In addition, avoiding the geometric relaxation between the lowest excited state (D1) and the ground state (D0), as well as reducing electron-vibration coupling and non-adiabatic coupling in the low-frequency region can effectively decrease the kIC constants. Our study proposes an innovative design approach aiming to develop stable and efficient NIR-II window neutral radical luminescent materials utilizing heteroatomic fused-rings as key elements.

Luminescent Radicals

Organic radicals are attracting increasing interest as a new class of molecular emitters. They demonstrate electronic excitation and relaxation dynamics based on their doublet or higher multiplet spin states, which are different from those based on singlet-triplet manifolds of conventional closed-shell molecules. Recent studies have disclosed luminescence properties and excited state dynamics unique to radicals, such as highly efficient electron-photon conversion in OLEDs, NIR emission, magnetoluminescence, an absence of heavy atom effect, and spin-dependent and spin-selective dynamics. These are difficult or sometimes impossible to achieve with closed-shell luminophores. This review focuses on luminescent organic radicals as an emerging photofunctional molecular system, and introduces the material developments, fundamental properties including luminescence, and photofunctions. Materials covered in this review range from monoradicals, radical oligomers, and radical polymers to metal complexes with radical ligands demonstrating radical-involved emission. In addition to stable radicals, transiently formed radicals generated in situ by external stimuli are introduced. This review shows that luminescent organic radicals have great potential to expand the chemical and spin spaces of luminescent molecular materials and thus broaden their applicability to photofunctional systems.