Electronic Properties and Electroluminescent OLED Performance of Panchromatic Emissive N-Aryl-2,3-naphthalimides

This report investigates the excited-state properties of a series of N-aryl-2,3-naphthalimides along with their fabrication into OLEDs and electroluminescence measurements. The N-aryl-2,3-NIs substituted specifically with chloro, fluoro, and methoxy substituents were chosen because of their unique propensity to display two emission bands or panchromatic fluorescence. Using the Lippert-Mataga analysis along with TD-DFT calculations, the excited states were determined to be n,π* and π,π*. The TD-DFT calculations on the geometries of the excited states indicate that the excited state shows a planar structure. The origin of both the short wavelength (SW) and long wavelength (LW) emission were correlated to specific geometries such that the SW emission originates from an "angled" structure in the excited state, and LW emission originates from an excited state of coplanar structure. All of the dyes investigated readily formed good films under ultrahigh vacuum deposition. The molecular energy levels of these compounds (HOMO and LUMO) were measured with cyclic voltammetry. Band gaps were also measured in both electrochemical and optical methods and indicate that the HOMOs of these fluorophores matched well with the anode (ITO work function), and their LUMOs matched well with the cathode (LiF/Al). To compare photoluminescence of the four dyes with their potential electroluminescence, three OLED devices were designed and fabricated. The electroluminescent spectra of these devices indicate that the panchromatic fluorescence, observed in solution, shifts toward the red in the solid-state. A plausible explanation appears to stem from an inability to inject electrons to the higher LUMO+1 orbitals; a process observed in the solution phase. Hence, the short wavelength fluorescence peak, a key component to panchromatic luminescence disappears in the OLED device. The observed EL spectrum from these smaller heteroatomic architectures is on par if not more broadly emissive than rubrene (5,6,11,12-tetraphenyltetracene), a red-colored C₄₂H₂₈ polycyclic aromatic hydrocarbon, that displays an orange-color EL.