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

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 (λmaxAbs: 294–320 nm), revealing low optical band gaps (2.77–3.79 eV). These materials emit blue fluorescence with λmaxEm 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.