Electrochemistry and Electrogenerated Chemiluminescence Processes of the Components of Aluminum Quinolate/Triarylamine, and Related Organic Light-Emitting Diodes

Abstract

Solution electrochemical studies have been conducted of the principle lumophores, dopants, and hole-transport agents of aluminum-quinolate(Alq3)-based organic light-emitting diodes (OLEDs) along with the characterization of their electrogenerated chemiluminescence (ECL). In acetonitrile/benzene solvent mixtures, Alq3 shows single one-electron reduction and oxidation processes, with a separation between the first oxidation and first reduction potentials, ΔEelectrochemical = 3.03 V, close to the estimates of energy difference between HOMO and LUMO levels obtained from absorbance spectra of thin films of Alq3, ΔEoptical = 3.17 eV. A new sulfonamide derivative of Alq3, (Al(qs)3), showed a positive shift (ca. 0.32 V) in the first reduction potential versus the parent molecule, and resolution of the overall reduction process into three successive, chemically reversible, one-electron reductions. Two successive one-electron oxidations are seen for 4,4‘-bis(m-tolyphenylamino)biphenyl (TPD), a hole-transporting material in many bilayer OLEDs, and for TPDF2, a fluorinated version of TPD, with TPDF2 oxidation occurring 0.1 V positive of that for TPD. Electrogenerated chemiluminescence reactions (Alq3-•/TPD+• (or TPDF2+•) and Al(qs)3-•/TPD+• (or TPDF2+•)) were found to produce emission spectra from Alq3*s or Al(qs)3*s states which were nearly identical to those seen from OLEDs based upon these molecules. Emission intensities increased with the increasing potential difference between the relevant redox couples. The diisoamyl derivative of quinacridone (DIQA), a quinacridone dopant for certain Alq3-based OLEDs, undergoes two successive one-electron reductions and two successive one-electron oxidations. The ECL reactions DIQA-•/DIQA+•, DIQA+•/Alq3-•, DIQA+•/Al(qs)3-•, DIQA-•/TPD+• and DIQA-•/TPDF2+• all produce the same singlet emissive state, DIQA*s, and the same emission spectral response seen in quinacridone and DIQA-doped OLEDs.