Spectroscopic mapping and selective electronic tuning of molecular orbitals in phosphorescent organometallic complexes - a new strategy for OLED materials

Abstract

The improvement of molecular electronic devices such as organic light-emitting diodes requires fundamental knowledge about the structural and electronic properties of the employed molecules as well as their interactions with neighboring molecules or interfaces. We show that highly resolved scanning tunneling microscopy (STM) and spectroscopy (STS) are powerful tools to correlate the electronic properties of phosphorescent complexes (i.e., triplet emitters) with their molecular structure as well as the local environment around a single molecule. We used spectroscopic mapping to visualize several occupied and unoccupied molecular frontier orbitals of Pt(II) complexes adsorbed on Au(111). The analysis showed that the molecules exhibit a peculiar localized strong hybridization that leads to partial depopulation of a dz² orbital, while the ligand orbitals are almost unchanged. We further found that substitution of functional groups at well-defined positions can alter specific molecular orbitals without influencing the others. The results open a path toward the tailored design of electronic and optical properties of triplet emitters by smart ligand substitution, which may improve the performance of future OLED devices.

Keywords: OLED; Pt(II) complex; charge transfer; density-functional theory; frontier orbitals; hybridization; scanning tunneling microscopy; scanning tunneling spectroscopy; triplet emitters.

Figure 1

Molecular structure of the complexes C1 to C4. In all cases the Pt atom is fourfold coordinated by N atoms, stemming from a tridendate ligand (TL, containing two triazole groups and one pyridine) and an ancillary ligand (AL, containing a pyridine group). The substituents R¹ to R³ are varied in order to investigate their influence on the adsorption as well as the electronic structure.

Figure 2

Topography analysis of a monolayer of C1 (a,b) and C2 (c–f) on Au(111). C1 grows in only one close-packed structure probably due to steric packing. C2 shows three different ordered structures, indicating the additional role of van der Waals forces between neighboring R³ alkyl chains for the self-assembly.

Figure 3

(a) Energy and LDOS of calculated orbitals for C1 in the gas phase. Here, a work function of 5.1 eV was assumed. This value results from minimizing the energy differences between calculated and measured energies of the HOMO and the LUMO, respectively. (b) Series of dI/dV maps (bottom) and corresponding schematic representation of energetic distribution (top). Molecular states related to the Pt d_z² orbital are colored in red, while ligand centered orbitals are grey-shaded. The arrows between (a) and (b) indicate the orbital shifts caused by the hybridization with the substrate states.

Figure 4

dI/dV maps of C1 at 1.4 V recorded in constant current (a) and constant height (b) mode, respectively. 83 pA were choosen as the current setpoint.

Figure 5

dI/dV spectra taken over the Pt atom and the pyridine group of C1 exhibit a peak at 1.9 V that we assign to the LUMO of the free molecule (see discussion in the text). For comparison, a spectrum of the bare Au(111) surface is also shown. Results for complex C2 are almost identical.

Figure 6

STM images of self-assembled monolayers of C3 (a) and C4 (b). Due to the rotational degree of freedom around the O–C bonds, the exact position of the methoxy group cannot be given here. Despite the different substituents R¹ and R² the complexes show similar packing structures indicated by the overlaid molecular models.

Figure 7

Series of dI/dV maps of complex C3 and corresponding calculated orbitals of gas-phase molecules.

Figure 8

Tunneling spectra of C3 and C4 each acquired at the pyridine and triazole groups of the TL. For comparison, the vertical lines indicate the HOMO and LUMO levels of C1 and C2 [30].