Theoretical design study on photophysical property of the organoboron quinolate derivatives

Abstract

The photophysical properties of organoboron quinolate derivatives can be modified readily by manipulating the coordination environment around the central boron atom. This class of compounds applied in organic light-emitting diodes (OLEDs) materials has been studied by quantum chemistry. To reveal the relationship between the structures and properties of these electroluminescent materials, the ground- and excited-state geometries were optimized at the B3LYP/6-31G(d) and CIS/6-31G(d) levels, respectively. The ionization potentials and electron affinities were computed. The mobilities of hole and electron in these compounds were studied computationally based on the Marcus electron transfer theory. The maximum absorption and emission wavelengths of these compounds were calculated using the time-dependent density functional theory method. The solvent effect on the absorption and emission wavelengths of these compounds was also considered by a polarizable continuum model. These results show that boron compounds which containing both the hydroxyquinoline/hydroxybenzoquinoline as ligand and O/S in position X follow the rule, that is, the emission shifts to longer wavelength as covalent nature of the boron–ligand bonding is increased. Meanwhile, the negative HOMO and IPs decrease but the negative LUMO and EAs increase by substitution of O with S in position X. It was deduced that both the hole- and electron-injection abilities are improved by substituting S in place of O in position X. After chemical modification in position R2 with electron-donating properties of NH2 or 1,4-diethynyl-2,5-dihexyloxybenzene, introduced 1,4-diethynyl-2,5-dihexyloxybenzene improves both the hole- and electron-transfer rate, which leads to better equilibrium property. It can be concluded that the better equilibrium property depends on the conjugated length of side chain in position R2. Moreover, exchanging the substituents R1 and R2 in BNO1a and BNO1’a can slightly change the hole-transfer rate by 0.04 eV. According to these calculations, series BNO and BNS can be applied as electron transport and hole transport materials at the same time. Specially, series BNO2 and BNS have better performance than Mes2B[p-4,4’-biphenyl-NPh(1-naphthyl)] (BNPB) in both the hole- and electron-injection ability.