Theoretical study on difunctional electroluminescent molecules containing aromatic heterocycle cores and hole-transporting triphenylamino units

Abstract

2,2′-difuryl-4,4′-(N,N,N′,N′-tetraphenyl) diaminobiphenyl (FurylBz-Ph4) and 5,5′-bis(4-N,N′- diphenylaminophenyl)-2,2′-bifuryl (PFDA-Ph4) are new light-emitting materials, which are capable of transporting holes. A series of new compounds whose structures were similar to that of FurylBz-Ph4 or PFDA-Ph4, were designed and calculated. The total energies, HOMO and LUMO orbital energies of these compounds at neutral, cationic and anionic states were obtained based on the optimised geometrical structures. The ionization potential ( I p) and electron affinity ( E a) were generated by means of density functional theory (DFT) calculated total energy of neutral, cationic and anionic states. The reorganization energy for the hole transport of compounds 1- 3, FurylBz-Ph4 and PFDA-Ph4 is close to that of TPD at the same level of calculation. For compounds 2, 3 and PFDA-Ph4, the order of I p is 3 > PFDA-Ph4 > 2. It is indicated that the contribution of aryl groups to I p follows the sequence: thienyl > furyl > pyrryl. The absorptions of these compounds had been also investegated by time-dependent density functional theory (TD-DFT) methods with 6-31G(d) basis set. The TD-DFT calculations indicate that the absorption maximum of compounds 2 is blue-shifted from PFDA-Ph4, while the absorption maximum of compounds 3 is red-shifted from PFDA-Ph4. Since the electron-withdrawing strength is in the order pyrryl > thienyl > furyl and PFDA-Ph4 has the longer conjugated length between the two amine nitrogen atoms than that of compound 2 and 3. The different bands of emission and reorganization energy can be obtained by changing the substituents and the site of substitution between two triphenylamino units.