*3.3.1 Pt(II) emitters with [O^N^C^N] ligands*

In 2013, Che et al. developed the first phosphorescent platinum(II) complexes supported by tetradentate [O^N^C^N] ligands for white OLED and polymer organic light-emitting diode (PLED) applications [36]. These complexes were found to possess desirable physical properties as OLED emitters including high thermal stability with Td > 400°C and ease of sublimation for vacuum deposition. Several follow-up studies on this family of Pt(II) emitters for high-efficiency OLEDs have been reported by the same group [22, 37–39].

### *3.3.1.1 Molecular design strategies*

In general, for monochromic blue to yellow OLEDs, emission from monomeric Pt(II) complexes should be dominant, and aggregate emission should be minimized for achieving a high color purity (**Figure 5**). Early works showed that platinum(II) [O^N^C^N] complexes are prone to excimeric emission at elevated concentration. In attempts to address the excimer issues for realizing monchromic green and yellow OLEDs, Che and co-workers proposed a strategy to bolster the 3D configuration of the moelcular strucutre of the complexes to supress the intermolecular interactions via the introduction of rigid and bulky substituents, such as t-Bu groups and a norbornene moiety, to the ligand periphery, and the incorporation of bridging tertiary arylamine units or biphenyl groups with spiro linkages to the ligand frameworks. These modifications were found to effectively disfavor intermolecular interactions, evident by the diminished emission self-quenching and excimeric emissions in solution and in thin film at high concentrations. In addition, the corresponding devices showed improved device efficiencies and diminished efficiency roll-offs. Additionally, Pt(II) [O^N^C^N] emitters bearing a cross-shaped molecular structure (i.e., a spiro linkage) may also cause molecular entanglement in the amorphous state, which help prevent recrystallization of the emissive layer and prolong operational lifetimes.

Through deliberate molecular design and variations in the doping concentration, the extent of intermolecular interactions and aggregations of platinum(II) [O^N^C^N] emitters could be controlled and manipulated for red and NIR as well as white OLED applications based on aggregation and monomer/aggregation emissions, respectively (**Figure 5**). Instead of tuning the 3D configuration to limit intermolecular interactions, for these applications, adopting a relatively planar ligand scaffold and introducing fluorine substituent(s) at specific position(s),

**173**

**Figure 6.**

*Chemical structures of platinum(II) complexes Pt-15–20.*

*Tetradentate Platinum(II) Emitters: Design Strategies, Photophysics, and OLED Applications*

which favor intermolecular π-π and/or Pt-Pt interactions for low-energy aggregate

The cyclometalated [O^N^C^N] ligand system is useful for the construction of robust and highly efficient Pt(II) emitters. Che et al. developed a panel of platinum(II) [O^N^C^N] emitters, i.e., **Pt-15**–**Pt-20** (**Figure 6**) [39], and systematically investigated their photophysical and electroluminescent properties, which

The intense absorption bands of **Pt-15**–**20** at <300 nm are assigned to intraligand π-π\* transitions, and the moderately intense bands at 430–450 nm with weak absorption at >460 nm are assigned to transitions with mixed MLCT and ILCT character. In degassed CH2Cl2 solutions, complexes **Pt-15**–**18** exhibit strong green to yellow luminescence (λem = 522–570 nm) with emission quantum yields and lifetimes in the range of 0.23–0.95 and 2.3–5.5 μs, respectively. With an additional t-Bu group at the *ortho*-position of the phenolate unit, **Pt-18** shows a much lower emission quantum yield (0.23) than **Pt-15**–**Pt-17** (0.77–0.95), indicating that the free rotation of this t-Bu group contributes to the increased non-radiative decay of the emissive state. Congeners **Pt-19** and **Pt-20**, with 6-5-6 metallacycles, are also highly efficient yellow (λem = 551 nm) and green (λem = 517 nm) phosphors, respectively. Their solution emission quantum yields (>0.80) and lifetimes (<5.1 μs) are similar to those of **Pt-15**–**17**. Femtosecond time-resolved fluorescence (fs-TRF) measurements suggested that **Pt-19** and **Pt-20** undergo an ultrafast intersystem crossing process with time constants of 0.44 and 0.15 ps, respectively. The emission

characters. Notably, **Pt-17**–**Pt-20** do not display excimer emissions in CH2Cl2 even at

sions as a low-energy band (ca. 650 nm) under the same conditions. This finding suggests that the incorporation of a bridging tertiary amine or a spiro-fluorene linkage in the ligand framework would be as effective in suppressing the intermolecular

MLCT and 3

M), while **Pt-15** and **Pt-16** showed excimer emis-

[L → π\*]

emission from the excited states of dimers or oligomers, are preferred.

*3.3.1.2 Photophysical properties and OLEDs based on monomer emission*

origin for **Pt-15**–**20** was assigned to excited states with mixed 3

*DOI: http://dx.doi.org/10.5772/intechopen.93221*

are summarized in **Tables 3** and **4**.

high concentrations (1.0 × 10<sup>−</sup><sup>4</sup>

**Figure 5.**

*Molecular design strategies for various OLED applications.*

*Tetradentate Platinum(II) Emitters: Design Strategies, Photophysics, and OLED Applications DOI: http://dx.doi.org/10.5772/intechopen.93221*

which favor intermolecular π-π and/or Pt-Pt interactions for low-energy aggregate emission from the excited states of dimers or oligomers, are preferred.
