NHC-derived carbon-centered luminescent radicals with short-wavelength emission via suppression of Kasha’s rule

Abstract

Luminescent organic radicals have garnered increasing attention owing to their versatile applications in sensing, imaging, and organic light-emitting diodes (OLEDs), attributed to their unique emission properties originating from the doublet spin state. However, the natural narrow bandgap of organic free radicals typically limits their emission to the long-wavelength region. Designing luminescent organic radicals with short-wavelength emission remains a significant challenge. Herein, a series of carbon-centered radicals with short-wavelength emission (383–476 nm) by combining N-heterocyclic carbenes with various polycyclic aromatic hydrocarbons (PAHs) (2-naphthyl, 2a^I and 2b^I; 2-phenanthryl, 2a^II and 2b^II; 2-anthryl, 2a^III and 2b^III; 3-phenanthryl, 2a^IV and 2b^IV). Theoretical calculations reveal that the introduction of PAHs significantly increases the ΔE_D2-D1 in 2a^I–III and 2b^I–III compared to that in phenyl-derived radical congeners. Consequently, the internal transition from D2 to D1 is impeded, leading to a high yield of D2 emission and a suppressed Kasha’s rule, thereby overcoming the limitations imposed by their narrow bandgap. For 2a^IV and 2b^IV, despite a moderately large ΔE_D2-D1 value, the ΔE_D3-D1 value exceeds 1 eV, indicating that their emission likely originates from the D3 state. Furthermore, we utilized 2a^III and 2b^III as emissive materials in OLEDs, resulting in blue emissions with external quantum efficiencies of 7.5% and 6.5%, respectively.