Novel multifunctional organic semiconductor materials based on 4,8-substituted 1,5-naphthyridine: synthesis, single crystal structures, opto-electrical properties and quantum chemistry calculation

Abstract

A series of 4,8-substituted 1,5-naphthyridines (1a–1h) have been successfully synthesised by a Suzuki cross-coupling between 4,8-dibromo-1,5-naphthyridine (4) and the corresponding boronic acids (2a–2h) in the presence of catalytic palladium acetate in yields of 41.4–75.8% and have ben well characterized. They are thermally robust with high phase transition temperatures (above 186 °C). Compounds 1b, 1e and 1f crystallized in the monoclinic crystal system with the space groups P2₁/c, P2₁/c and P2₁/n, respectively. All of them show the lowest energy absorption bands (λ_max^Abs: 294–320 nm), revealing low optical band gaps (2.77–3.79 eV). These materials emit blue fluorescence with λ_max^Em ranging from 434–521 nm in dilute solution in dichloromethane and 400–501 nm in the solid state. 4,8-Substituted 1,5-naphthyridines 1a–1h have estimated electron affinities (EA) of (2.38–2.72 eV) suitable for electron-transport materials and ionization potentials (IP) of 4.85–5.04 eV facilitate excellent hole-injecting/hole-transport materials properties. Quantum chemical calculations using DFT B3LYP/6-31G* showed nearly identical the lowest unoccupied molecular orbitals (LUMO) of −2.39 to −2.19 eV and the highest occupied molecular orbitals (HOMO) of −5.33 to −6.84 eV. These results demonstrate the 4,8-substituted 1,5-naphthyridines 1a–1h with a simple architecture might be promising blue-emitting (or blue-green-emitting) materials, electron-transport materials and hole-injecting/hole-transport materials for applications for developing high-efficiency OLEDs.