Structural, electronic, and optical properties of phosphole-containing pi-conjugated oligomers for light-emitting diodes

The purpose of this work is to provide an in-depth interpretation of the optical and electronic properties of a series of phosphole derivatives, including 2,5-diphenylthiooxophosphole (2a), 2-phenyl-5-biphenylthiooxophosphole (3a), 2-phenyl-5-stilbenylthiooxophosphole (4a), 2,5-dithienylthiooxophosphole (2b), 2-thienyl-5-biphenylthiooxophosphole (3b), 2-thienyl-5-stilbenylthiooxophosphole (4b), and dibenzophosphole 1. These thiooxophospholes show great potential for application in OLEDs as efficient red emitters due to the tuning of the optical and electronic properties by the use of various substituents at the 2,5-positions of the phosphole ring. The geometric and electronic structures of the oligomers in the ground state were investigated using density functional theory (DFT) and the ab initio HF, whereas the lowest singlet excited states were optimized with ab initio CIS. To assign the absorption and emission peaks observed in the experiment, we computed the energies of the lowest singlet excited states with time-dependent DFT (TD-DFT). All DFT calculations were performed using the B3LYP functional and the 6-31G (d) basis set. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials, and electron affinities for the phosphole derivatives are significantly affected by varying the phosphole ring substituents at the 2,5-positions, which favor the hole and electron injection into OLEDs. The absorption and emission spectra exhibit red shifts to some extent the absorption spectra: 339.63 (1)<358.65 (2a)<373.77 (3a)<443.89 nm (4a) and 403.03 (3b)<449.11 (2b)<460.19 nm (4b); the emission spectra: 418.42 (1)<513.62 (2a)<556.51 (3a)<642.59 nm (4a) and 568.31 (2b)<631.11 (3b)<647.35 nm (4b)] and the Stokes shifts are unexpectedly large ranging from 78 to 228 nm resulting from a more planar conformation of the excited state for the phosphole derivatives.