**3. Tetradentate platinum(II) emitters**

#### **3.1 Platinum(II) porphyrin complexes**

The first photophysical studies of Pt(II) porphyrins were reported in the early 1970s and were triggered by the physicochemical relevance of metalloporphyrins such as chlorophyll, haem, and vitamin B12, which serve key biological functions [23]. This class of complexes is known for their high stability against heat, solvents, and extreme pH as a result of the strong coordination of Pt(II) ions in rigid porphyrin scaffolds. Pt(II) porphyrins exhibit intense absorptions in the visible region, and their electronic spectra are characterized by a Soret band at approximately 400 nm and two Q bands between 500 and 600 nm, which are attributed to porphyrin-centered <sup>1</sup> π-π\* electronic transitions. The triplet formation yields for Pt(II) porphyrins were reported to be close to unity due to an ultrafast intersystem crossing process that occurs on a sub-ps time scale [24]. These complexes typically display strong saturated red to near infrared (NIR) phosphorescence, depending on the structures of macrocycles, with long decay times of tens of microseconds under anaerobic conditions because the emissive excited state is essentially <sup>3</sup> LC (3 π, π\* ) in nature localized in the porphyrin ligand. For this reason, the emission properties can be rationally tuned by modifying the porphyrin ligands.

In 1998, Thompson and Forest reported the first use of a Pt(II) porphyrin complex, **Pt-1**, in electrophosphorescent devices (**Figure 2**) [18], which generated saturated red EL emission at 650 nm with a peak internal quantum efficiency (IQE) of 23%. Since then, the development of Pt(II) porphyrin complexes as phosphorescent emitters has attracted considerable interest [25–27].

Che et al. found that with the introduction of electron-deficient pentafluorophenyl rings at the meso positions of the porphyrin scaffold, **Pt-2** exhibited superior stability against oxidative degradation relative to that of the parental complex [Pt(TPP)] (TPP = 5,10,15,20-tetraphenylporphyrinato) (**Figure 2**) [28]. Saturated red (λmax = 647 nm) porphyrin-centered phosphorescence with a long lifetime (τem = 60 μs) was observed for **Pt-2** in CH2Cl2 at room temperature. Saturated red OLED devices with different doping concentrations of **Pt-2** have

**167**

**Figure 2.**

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

been characterized. The EL spectra showed emission maxima at 655 nm, similar to that of the photoluminescent (PL) spectrum. At low driving voltages, emission from the host was not detected, indicating nearly complete energy transfer from the host. When a high voltage was applied, a blue emission from the host was observed, and its intensity increased with the driving voltage, which could be attributed to the saturation effect due to the long emission decay time of **Pt-2**. The efficiency increased until the dopant ratio reached 8% but decreased when the ratio was increased beyond 8%, which was rationalized by intermolecular quenching

Wang and co-workers designed a group of platinum(II) porphyrin dendrimers (**Pt-3**–**5**) [29] with different alkyl chain lengths to adjust the distance between the Pt porphyrin core (the emissive center) and side carbazole groups (hole- and energy-transfer fragments) (**Figure 2**) and systematically investigated intra- and intermolecular energy-transfer mechanisms. In solution, both fluorescence at 357 nm from the carbazole moieties and phosphorescence at 660 nm from the Pt(II) porphyrin core were observed. The carbazole emission disappeared when doped in a solid matrix, indicating an efficient energy-transfer process. Intramolecular energy-transfer was facilitated by reducing the distance between the emitting core and the side carbazole groups. Solution-processed EL devices based on **Pt-3** were fabricated with a structure of [ITO/poly(styrenesulfonate)-doped poly(3,4-ethyle nedioxythiophene)/**Pt-3**/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/ tris-(8-hydroxyquinoline)aluminum (Alq3)/Ca/Al] to evaluate the optoelectronic

660, and 720 nm was observed. The strong vibronic emission peaked at 660 was attributed to phosphorescence from the Pt(II) porphyrin cores, while the minor high-energy emission bands at 540 and 590 nm were suggested to be derived from thermally populated triplet states ("hot bands") or singlet states generated by TTA.

) with emission maxima at 540, 590,

processes due to the high density of triplet excitons.

*Chemical structures of platinum(II) porphyrin complexes Pt-1–8.*

properties. Bright phosphorescence (600 cd/m2

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

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

**Figure 2.** *Chemical structures of platinum(II) porphyrin complexes Pt-1–8.*

been characterized. The EL spectra showed emission maxima at 655 nm, similar to that of the photoluminescent (PL) spectrum. At low driving voltages, emission from the host was not detected, indicating nearly complete energy transfer from the host. When a high voltage was applied, a blue emission from the host was observed, and its intensity increased with the driving voltage, which could be attributed to the saturation effect due to the long emission decay time of **Pt-2**. The efficiency increased until the dopant ratio reached 8% but decreased when the ratio was increased beyond 8%, which was rationalized by intermolecular quenching processes due to the high density of triplet excitons.

Wang and co-workers designed a group of platinum(II) porphyrin dendrimers (**Pt-3**–**5**) [29] with different alkyl chain lengths to adjust the distance between the Pt porphyrin core (the emissive center) and side carbazole groups (hole- and energy-transfer fragments) (**Figure 2**) and systematically investigated intra- and intermolecular energy-transfer mechanisms. In solution, both fluorescence at 357 nm from the carbazole moieties and phosphorescence at 660 nm from the Pt(II) porphyrin core were observed. The carbazole emission disappeared when doped in a solid matrix, indicating an efficient energy-transfer process. Intramolecular energy-transfer was facilitated by reducing the distance between the emitting core and the side carbazole groups. Solution-processed EL devices based on **Pt-3** were fabricated with a structure of [ITO/poly(styrenesulfonate)-doped poly(3,4-ethyle nedioxythiophene)/**Pt-3**/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/ tris-(8-hydroxyquinoline)aluminum (Alq3)/Ca/Al] to evaluate the optoelectronic properties. Bright phosphorescence (600 cd/m2 ) with emission maxima at 540, 590, 660, and 720 nm was observed. The strong vibronic emission peaked at 660 was attributed to phosphorescence from the Pt(II) porphyrin cores, while the minor high-energy emission bands at 540 and 590 nm were suggested to be derived from thermally populated triplet states ("hot bands") or singlet states generated by TTA.

*Liquid Crystals and Display Technology*

butions are not mentioned herein.

to porphyrin-centered <sup>1</sup>

**3. Tetradentate platinum(II) emitters**

**3.1 Platinum(II) porphyrin complexes**

quenching (ACQ ). Therefore, the appropriate management of intermolecular interactions/aggregation by modulating the 3D morphology and electromagnetic properties of the complexes is crucial in the molecular design of Pt(II) emitters for specific OLED applications in order to achieve optimum device performances (i.e.,

In the context of ligand architecture, the employment of tetradentate ligands in the design of platinum(II) emitters has clear advantages in terms of both chemical and thermal stabilities and luminescent efficiency compared to their bidentate and tridentate ligand counterparts. Tetradentate ligands provide a more stable scaffold for the coordination of platinum by offering the strong chelation effect, which could suppress ligand dissociation and demetalation. In addition, the rigid tetradentate ligand framework could largely restrict the excited-state metal-ligand structural distortion that in turn facilitates radiative deactivation of the emissive

high color purity, device efficiency, and long operational lifetime).

excited state, boosting the emission quantum yield of the Pt(II) emitter.

This chapter aims to provide an overview of mononuclear Pt(II) emitters containing tetradentate ligands reported in the literature. To keep the chapter to a reasonable size, we restrict our discussions to several selected classes of tetradentate platinum(II) emitters and apologize to the contributors to this field whose contri-

The first photophysical studies of Pt(II) porphyrins were reported in the early 1970s and were triggered by the physicochemical relevance of metalloporphyrins such as chlorophyll, haem, and vitamin B12, which serve key biological functions [23]. This class of complexes is known for their high stability against heat, solvents, and extreme pH as a result of the strong coordination of Pt(II) ions in rigid porphyrin scaffolds. Pt(II) porphyrins exhibit intense absorptions in the visible region, and their electronic spectra are characterized by a Soret band at approximately 400 nm and two Q bands between 500 and 600 nm, which are attributed

Pt(II) porphyrins were reported to be close to unity due to an ultrafast intersystem crossing process that occurs on a sub-ps time scale [24]. These complexes typically display strong saturated red to near infrared (NIR) phosphorescence, depending on the structures of macrocycles, with long decay times of tens of microseconds under

nature localized in the porphyrin ligand. For this reason, the emission properties

In 1998, Thompson and Forest reported the first use of a Pt(II) porphyrin complex, **Pt-1**, in electrophosphorescent devices (**Figure 2**) [18], which generated saturated red EL emission at 650 nm with a peak internal quantum efficiency (IQE) of 23%. Since then, the development of Pt(II) porphyrin complexes as phosphores-

Che et al. found that with the introduction of electron-deficient pentafluorophenyl rings at the meso positions of the porphyrin scaffold, **Pt-2** exhibited superior stability against oxidative degradation relative to that of the parental complex [Pt(TPP)] (TPP = 5,10,15,20-tetraphenylporphyrinato) (**Figure 2**) [28]. Saturated red (λmax = 647 nm) porphyrin-centered phosphorescence with a long lifetime (τem = 60 μs) was observed for **Pt-2** in CH2Cl2 at room temperature. Saturated red OLED devices with different doping concentrations of **Pt-2** have

anaerobic conditions because the emissive excited state is essentially <sup>3</sup>

can be rationally tuned by modifying the porphyrin ligands.

cent emitters has attracted considerable interest [25–27].

π-π\* electronic transitions. The triplet formation yields for

LC (3

π, π\* ) in

**166**

The data from devices prepared by spin coating these dendrimers as the non-doped emissive layer are better than those of small molecule Pt(II) porphyrin analogs.

The emission of platinum(II) porphyrin complexes can be further shifted to the NIR region by extending the π conjugation of the porphyrin ligand [30]. Schanze and co-workers developed a series of NIR-emitting platinum(II) di- and tetrasubstituted benzoporphyrin complexes, **Pt-6**–**8** (**Figure 2**), to investigate structureproperty relationships and the link between photophysical parameters and OLED performances. The photophysical parameters are listed in **Table 1**. Although the di-substituted porphyrin complex gives a higher ФPL (0.49) and longer τem (53.0 μs) than the tetra-substituted complexes (ФPL = 0.33–0.35; τem = 29.9–32.0 μs) in solution, the same trend does not hold in solid matrices, in which the emission lifetimes of **Pt-6**–**8** are comparable (45.7–57.5 μs). The EQEs of all the devices (8.0–9.2%) were similar. Consequently, the authors concluded that (i) the major non-radiative decays are associated with out-of-plane distortion of the porphyrin ligands, (ii) the rigid polymer matrix could help suppress the enhanced non-radiative decay of the more distorted complexes (**Pt-6** and **Pt-7**) in solid matrices, and (iii) the performance of platinum(II) benzoporphyrin-based OLEDs was observed to strongly correlate with the emission lifetime in the solid matrix.

Pt(II) porphyrins as OLED emitters generally exhibit high thermal stability and outstanding performance in saturated red and NIR devices in terms of color purity and EL efficiencies attributed to their narrow emission band and high emission quantum yields. Nevertheless, the practical interest of this class of Pt(II) emitters is limited by the long emission decay times, which would result in substantial efficiency loss at higher luminance. It is hypothesized that careful device design and the use of appropriate auxiliary materials to mitigate TTA, e.g., by using a double host to broaden the recombination zone, could be a strategy for improving the practicality of these materials.
