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Review

Emission Wavelength Control via Molecular Structure Design of Dinuclear Pt(II) Complexes: Optimizing Optical Properties for Red- and Near-Infrared Emissions

by
Hea Jung Park
Department of Chemistry and School of Interdisciplinary Natural Science with Flexible Major, Changwon National University, Changwon 51140, Republic of Korea
Crystals 2025, 15(3), 273; https://doi.org/10.3390/cryst15030273
Submission received: 15 February 2025 / Revised: 11 March 2025 / Accepted: 13 March 2025 / Published: 15 March 2025
(This article belongs to the Special Issue Feature Papers on “Hybrid and Composite Crystalline Materials” 2025)
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Abstract

:
Phosphorescent Pt(II) complexes have garnered significant attention as key components in luminescence-based systems due to their highly efficient emission properties. A notable characteristic of these complexes is their ability to form excimers through strong molecular stacking in concentrated solutions or solid film states. This aggregation-driven emission, primarily arising from metal–metal to ligand charge transfer (MMLCT), is influenced by overlapping d-orbitals oriented perpendicular to the square planar structure of the Pt(II) complexes. Although this property hinders the development of pure blue-emitting Pt(II) complexes, it facilitates the design of materials that emit red- and near-infrared (NIR) light. By employing advanced molecular design techniques, dinuclear Pt(II) complexes have been optimized to significantly enhance red and NIR emissions through the modulation of Pt-Pt interactions and adjustments in ligand electron densities. This review elucidates how the control of Pt-Pt distances and strategic ligand modifications can directly influence the emission spectra toward red and NIR regions. A comparative analysis of recent studies underscores the novelty and effectiveness of double-decker-type dinuclear Pt(II) complexes in achieving efficient emission characteristics in the long-wavelength range. These insights may guide the design of molecular structures for next-generation organometallic phosphorescent materials.

1. Introduction

The demand for phosphorescent materials in red- and near-infrared (NIR) spectral regions has increased due to their applications in OLEDs, optical sensors, biological imaging, and phototherapy [1,2,3,4,5,6,7,8,9]. Transition metal-based organometallic compounds, such as Ir(III), Pt(II), and Os(II) complexes, have been extensively studied as efficient emissive materials. Among these, Pt(II)-based red- and near-infrared (NIR) emitters have garnered significant attention due to their advantageous photophysical properties [10,11,12]. Pt(II) complexes easily form excimers due to their square planar structure, which facilitates molecular aggregation or stacking in highly concentrated solutions or solid films [13,14,15,16,17] (Figure 1). These materials facilitate efficient emissions in the red and NIR spectral regions and exhibit strong metal–metal to ligand charge transfer (MMLCT) characteristics, attributed to the overlap of vacant dz2 orbitals between adjacent Pt centers [13]. This interaction enables short excited-state lifetimes and high luminescence efficiency. Additionally, red and NIR emissions can be achieved without excessively extending the π-conjugation length of the organic ligand, resulting in a lower molecular weight. This provides an advantage when used as an emissive material in the emitting layer of OLEDs fabricated via vacuum deposition. Numerous studies have been conducted on crystalline and polymeric Pt(II) complexes characterized by pronounced intermolecular Pt−Pt interactions [18,19,20,21,22]. In particular, in the solid state, specific monomeric bidentate Pt(II) complexes exhibit unexpected red or NIR emissions that are attributable to aggregation-induced excimer formation [23,24,25]. However, apart from using Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) to determine molecular packing distances, verifying the precise structures of these excimers in the solid state remains challenging. Therefore, the development of novel NIR-emissive Pt(II) complexes necessitates a strategy based on rational molecular design. One approach to designing reliable and predictable phosphors involves clearly defining the structures of the Pt(II) complexes, enabling intermetallic interactions within Pt−Pt distances that are reasonably governed by the characteristics of the ligands.
Recent research on dinuclear Pt(II) complexes, which incorporate two Pt centers within a single molecule, is progressing rapidly. To precisely modulate their optical properties, diverse molecular design strategies are essential, including ligand modifications, electron density tuning, and Pt-Pt distance regulation. In the study of dinuclear Pt(II) complexes, two main types of molecular structures are distinguished by the nature of the platinum interactions within each molecule: (1) structures where two Pt metals are aligned parallel to each other without orbital interactions, (Figure 2a) and (2) structures where the empty dz2 orbitals of the two Pt metals overlap significantly (Figure 2b). The focus of recent research has shifted towards the latter, known as the double-decker type (or “half-lantern” type), especially for producing emissions in the red and NIR spectra. This type of dinuclear Pt(II) complex consists of two types of ligands: cyclometalating and bridging ligands.
Unlike the excimer forms caused by molecular aggregation in concentrated liquid or solid states, designing structures to enforce the overlap of dz2 orbitals is beneficial (Figure 2b). This approach not only ensures the reproducibility of dinuclear molecular structures but also allows the systematic analysis of the optical properties of red- or NIR-emissive dinuclear Pt(II) complexes. The overlap of dz2 orbitals between two Pt(II) metal centers leads to the formation of new dσ and dσ* orbitals (Figure 3). As the two Pt metals come closer, the energy state of the dσ* orbital, which becomes the highest occupied molecular orbital (HOMO), increases, reducing the optical bandgap. Consequently, this induces a narrow optical bandgap suitable for red and NIR emissions. Typically, the π* orbital of the chromophoric cyclometalating ligand is positioned between the dσ* orbital and the vacant 6pσ orbital [26,27], and the emission process follows an MMLCT mechanism.
Recently, many researchers have been actively studying double-decker-type dinuclear Pt(II) complexes as emitting materials due to their advantages [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70]; however, there is still a lack of in-depth research on their optical properties from a molecular perspective. To control the emission wavelength by manipulating molecular structures, it is necessary to adjust the HOMO and LUMO energy levels. However, the factors influencing these energy levels are more complex in dinuclear Pt(II) complexes compared to their mononuclear counterparts. In mononuclear Pt(II) complexes, the energy gap is primarily controlled by the structure of the organic ligand, as in other luminescent organometallic complexes [71,72,73]. In contrast, for double-decker-type dinuclear Pt(II) complexes, multiple factors must be considered simultaneously. These include not only the conjugation length of the chromophoric ligands and their electron density but also the Pt-Pt distance within the molecule, all of which influence the energy gap. This complexity makes it challenging to systematically control the emission properties of the dinuclear Pt(II) complexes. This review paper exclusively examines the modulation of emission wavelengths and the optical characteristics of double-decker-type dinuclear Pt(II) complexes in the red-to-NIR spectral range, drawing on recent investigations. By categorizing and analyzing the factors that influence their optical bandgaps, this study aims to provide a structural design guideline for optimizing the optical properties of double-decker-type dinuclear Pt(II) complexes.

2. Emission Wavelength Control

2.1. Cyclometalating Ligand Control

2.1.1. π-Conjugation Length Extension

The simplest approach to achieving red and NIR emissions in organometallic compounds is to extend the π-conjugation length of the cyclometalating ligand, thereby facilitating long-wavelength emissions [74,75,76,77,78]. A study on Pt(II) complexes using porphyrin derivatives as ligands demonstrated that the emission wavelength shifted toward longer wavelengths as the π-conjugation length of the porphyrin was extended (Figure 4) [79].
In the case of dinuclear Pt(II) complexes, some reported compounds feature an extended π-conjugation of ligands. Pt-1 and Pt-2 [80] incorporated ligands with a donor–acceptor (D-A) framework, resulting in effective, red-shifted emissions due to intense intramolecular charge transfer (ICT) (Figure 5a). Both Pt-1 and Pt-2 showed similar red emissions at 690 nm in the DCM solution (Figure 5b). However, in the case of these Pt(II) complexes, effective Pt-Pt orbital overlap does not occur due to the limited angle caused by the pyrazole bridging ligand, resulting in the excited state being primarily characterized by 1LC and 3MLCT transitions, with minimal contribution from 3MMLCT. Consequently, additional emissions were observed at a shorter wavelength of 480 nm.
Recently, in 2024, Zhang et al. reported Pt-3, which incorporates a D-π-A-type cyclometalating ligand (C^N) to develop new deep-red and NIR-emissive dinuclear Pt(II) complexes [81]. In this complex, 4-(tert-butyl)-N-(4-(tert-butyl)phenyl)-N-phenylaniline was introduced as the donor (D) unit, naphthalene was introduced as the π-bridge, and isoquinoline was introduced as the acceptor (A) unit (Figure 6a). A π-bridge was introduced into the cyclometalating ligand to extend the π-conjugation, and the resulting Pt-3 exhibited maximum emission wavelengths of 686 nm in solution and 704 nm in the neat film state. However, in double-decker-type dinuclear Pt(II) complexes, the π-conjugation length of the ligand is not strictly proportional to the emission maximum wavelength. Simply increasing the π-conjugation length has limitations in inducing emissions in the deep-red and NIR regions, primarily because the HOMO originates from Pt dσ* orbitals. This means that not only the π-conjugation length but also the Pt-Pt distance influences the HOMO-LUMO energy gap. Therefore, merely extending the π-conjugation length may not effectively induce luminescence in these regions.

2.1.2. Electron Density Modulation

In double-decker-type dinuclear Pt(II) complexes, the cyclometalating ligands that bond to the Pt(II) metal in a square plane exist perpendicular to the Pt-Pt orbital overlap. Therefore, it has been considered that the electron density of the ligands in these compounds has little influence on the Pt-Pt distance. In 2022, Park et al. synthesized and analyzed the optical properties of five dinuclear Pt(II) complexes (Pt-4–Pt-8) incorporating phenylpyridine-type C^N ligands [82] (Figure 7). The synthesized Pt(II) complexes in this study exhibited maximum emissions in the deep-red and NIR regions at 659–802 nm. Compared with the previously reported Pt(II) complexes using organic ligands with extended π-conjugation lengths [80,81], Pt-4–Pt-8 showed emissions in a longer wavelength region, even though they had shorter π-conjugation. This result clearly demonstrates that there are limitations in inducing a narrow bandgap for NIR emissions by relying on extending the π-conjugation length of organic ligands. Single-crystal data confirmed that the Pt-Pt distance in the electron-deficient pyrazine-incorporated complex (Pt-7, dPt−Pt = 2.921 Å) was shorter than that in the pyridine-incorporated complex (Pt-6, dPt−Pt = 2.968 Å) (Figure 8). Notably, Pt-7, which exhibited the shortest Pt-Pt distance, demonstrated maximum emissions at 802 nm in the NIR region. Crystallographic data revealed that the introduction of a pyrazine moiety, which is electron-deficient compared to pyridine, into the organic ligand reduces the Pt(II)-N distance. This shortening is attributed to increased π-backdonation (Figure 9a). Simultaneously, the Pt-Pt distance also decreases. This phenomenon can be elucidated by combining it with Density Functional Theory (DFT) results, which suggest that enhanced π-backdonation from the filled Pt dxz orbital to the empty π* orbital of the ligand reduces the electronic repulsion between the partially overlapping dxz orbitals of the two Pt metals, resulting in a decrease in the Pt-Pt distance (Figure 9b). This suggests that tuning the electron density of C^N ligands can serve as an effective strategy for modulating the Pt-Pt distance, thereby facilitating long-wavelength emissions by reducing the Pt-Pt distance.

2.1.3. Isomeric Control

Cyclometalating ligands can adopt either symmetric or asymmetric structures. In the case of asymmetric structures, the double-decker-type dinuclear Pt(II) complex can exist as syn- or anti-form isomers. The syn(cis)- and anti(trans)-configurations are determined by the coordination geometry of the two cyclometalating ligands, which are arranged in parallel within a double-decker configuration of the molecule. It has been reported that these isomeric forms are primarily induced by variations in the coordination pattern of the bridging ligand rather than by direct modifications to the cyclometalating ligand [45,83,84]. In their 2024 study, Yao et al. reported the synthesis of syn- and anti-isomers of dinuclear Pt(II) complexes through the incorporation of various bridging ligands, followed by their isolation and a comprehensive analysis of their crystal structures and photophysical properties [27] (Figure 10). In this study, the syn-isomer exhibited a smaller energy gap than the anti-isomer, resulting in a red-shifted emission wavelength despite having a longer intramolecular Pt-Pt distance. The Pt-9-syn and Pt-10-syn compounds exhibit longer emission wavelengths of 801 nm and 657 nm, respectively, compared to their anti-counterparts, which emit at 670 nm (Pt-9-anti) and 629 nm (Pt-10-anti). According to the results of their DFT calculations, Pt-9-anti (and Pt-10-anti) exhibit a HOMO distribution similar to that of Pt-9-syn (and Pt-10-syn), mainly localized on the antibonding Pt-Pt dσ* orbitals and slightly on the carbazole moiety of the ligand. However, the LUMO distribution of Pt-9-anti (and Pt-10-anti) is primarily localized on one pyrimidine moiety of the two ligands, while in Pt-9-syn (and Pt-10-syn), it is localized on both pyrimidine moieties. The resulting change in the LUMO energy level appears to have a more significant impact on the energy gap than changes in the distance between the Pt metals.
A similar phenomenon was previously reported by M. Kato et al. in 2008, where the complex [Pt2(bpy)2(pyt)2][PF6]2 (bpy = 2,2′-bipyridine, pty = pyridine-2-thiolate ion) exhibited distinct emission characteristics depending on its isomeric form [85]. The anti-isomer demonstrated a maximum emission at 603 nm, whereas the syn-isomer displayed a significantly red-shifted maximum emission at 766 nm in the near-infrared region. This difference in energy bandgaps between isomers is analogous to the phenomenon observed in Ir(III) complexes, where the meridional isomer exhibits a smaller bandgap compared to the facial isomer.
Additionally, a 2019 study by Xiong et al. demonstrated that the introduction of 1-naphthyl-1-isoquinoline (niq), 2-naphthyl-isoquinolinate (2niq), 1-naphthyl-2-quinoline (nq), and 2-naphthyl-2-quinoline (2nq) isomers as C^N ligands in dinuclear Pt(II) complexes led to variations in emission wavelengths (Figure 11) [86]. These changes are dependent on the planarity of the ligands, as confirmed by single-crystal data. Complexes incorporating the highly planar niq and 2niq C^N ligands, namely Pt-11 (λmax = 677 nm) and Pt-13 (λmax = 693 nm), exhibited red-shifted emissions compared to those with less planar nq and 2nq ligands, Pt-12 (λmax = 659 nm) and Pt-14 (λmax = 689 nm), respectively. These findings suggest that the bandgap energy changes in dinuclear Pt(II) complexes are influenced by both structural alterations induced by the planarity of the C^N ligands and shifts in the HOMO and LUMO energy levels, which arise from differences in π-conjugation positions when using isomeric C^N ligands with identical atomic compositions and π-conjugation lengths.

2.1.4. Substituent Effects

Substituents attached to the cyclometalating ligand also influence the emission wavelength. According to a 2024 study by Baskaran et al., the incorporation of substituents with similar steric bulk, such as methyl (-CH3) and trifluoromethyl (-CF3) groups at the same position, resulted in variations in Pt-Pt distances (Figure 12a) [10]. The electron-donating CH3-substituted complex (Pt-15, dPt-Pt = 2.903 Å) exhibited a longer Pt-Pt distance compared to the electron-withdrawing CF3-substituted complex (Pt-16, dPt-Pt = 2.883 Å). Owing to the decreased Pt-Pt distance in Pt-16, it displayed a peak emission maximum at 725 nm within the NIR region, which is a longer wavelength compared to that of Pt-15 (λmax = 705 nm) (Figure 12b). The authors attributed these results to the electron-withdrawing group (EWG) rendering the C^N ligand more electron-deficient, thereby enhancing π-backdonation and consequently reducing the Pt-Pt distance. This suggests that emission wavelength modulation in these complexes is primarily driven by changes in the Pt-Pt distance induced through ligand electron density tuning.
Additionally, an increase in the flexibility of the cyclometalating ligand resulted in a non-planar, wavy molecular structure, leading to an increased Pt-Pt distance. According to a 2024 study by Esteruelas et al. on Pt-18, which featured an aromatic ring in the ligand connected via an oxygen atom, it exhibited a more pronounced wavy structure compared to its counterpart, Pt-17, which lacks this feature [87] (Figure 13). This structural distortion resulted in an extended Pt-Pt distance from 3.0515 Å in Pt-17 to 3.2689 Å in Pt-18. Pt-17 exhibited a maximum emission at 670 nm in solution and at 660 nm in film, whereas Pt-18 showed practically no emission (Figure 13). Furthermore, the use of a tridentate (N^C^N) ligand instead of a bidentate (C^N) ligand allowed for the incorporation of only a single bridging ligand within the dinuclear Pt(II) complex. Compared to structures containing two bridging ligands, this configuration increased the molecular flexibility, thereby providing greater potential for an extended Pt-Pt distance. Furthermore, molecules with structural flexibility are more susceptible to increased interactions between halogen(X)-containing solvent molecules and the aromatic moieties of the ligand, known as π-X interactions, in dilute solutions. These interactions readily lead to an expanded Pt-Pt distance, thereby disrupting metal–metal-to-ligand charge transfer (MMLCT). Consequently, as the concentration of Pt-17 decreases in 1,2-dichloroethane, the MMLCT characteristic weakens, and the relative emission intensity at shorter wavelengths around 480 nm increases (Figure 14b).

2.2. Bridging Ligand Control

2.2.1. Biting Angle Effect

Bridging ligands rarely have a direct impact on the HOMO and LUMO energy levels of double-decker-type dinuclear Pt(II) complexes. However, they significantly influence the Pt-Pt distance and molecular rigidity, which, in turn, play a crucial role in determining the emission wavelength and photoluminescence efficiency. In double-decker-type dinuclear Pt(II) complexes, the biting angle of the bridging ligand is critical for facilitating effective Pt-Pt orbital overlap [27,88]. Typically, a four-bond bridge mode promotes more efficient orbital interactions between the Pt centers than a three-bond bridge mode [89] (Figure 15). This enhanced efficiency is attributed to the closer proximity and linear arrangement of the orbitals in the four-bond bridge mode, which facilitates greater orbital overlap and, consequently, results in a shorter Pt-Pt distance.
Bright red emission has been achieved using four-bond bridging ligands such as mercapto- [86,90,91,92] or hydroxy- [53,93] substituted N-heterocycles (N-C-S or N-C-O). More recently, rigid and fused N-heterocycles, including N-protonated α-carboline [88], 7-azaindole, and indolo[2,3-b]indole [87] (N-C-N), have also been explored as bridging ligands (Figure 16).
However, even in the case of a three-bond bridging ligand, such as pyrazole, the introduction of bulky substituents on bridging ligands can induce steric repulsion, effectively reducing the Pt-Pt distance and thereby shifting the emission wavelength toward a longer wavelength region. In 2005, B. Ma et al. reported on Pt-Pt distance and luminescence characteristics by introducing various substituents with different levels of steric bulkiness to pyrazole, which is a three-bond bridging ligand [52] (Figure 17). The Pt-Pt distance decreases in the order of Pt-19, Pt-20, Pt-21, and Pt-22 as the substituents of the bridging ligands increase in steric bulkiness from H to CH3 to t-Bu. Among the synthesized Pt(II) complexes, Pt-22, which had the highest number of bulky substituents, exhibited the shortest Pt-Pt distance of 2.834 Å and consequently showed emissions at the longest wavelength, 570 nm. This is because the two cyclometalating ligands became closer to each other due to the steric repulsion between the substituents of the bridging ligands and the cyclometalating ligands. Although the dinuclear Pt(II) compounds reported in this study did not possess deep-red and NIR emissions, the authors reported that the bandgap narrowed, and the emission wavelength shifted to the long wavelength region as the Pt-Pt distance decreased. This suggests that the biting angle can be affected by the bridging ligand and its substituents, which can change the Pt-Pt distance (Figure 18).

2.2.2. Electronic Effects

According to a 2023 study by Rajakannu et al., the Pt-Pt distance can be modulated by adjusting the electron density of the bridging ligand [94]. Specifically, replacing oxygen (-O) in the bridging ligand with less electronegative sulfur (-S) destabilizes the metal-centered HOMO energy level, thereby narrowing the HOMO-LUMO energy gap. Consequently, this reduction in the energy gap facilitates emissions at longer wavelengths. The authors synthesized and systematically analyzed the optical properties of six compounds (Pt-23–Pt-28) incorporating two types of bridging ligands: benzothiazole-2-thiol and benzoxazole-2-thiol (Figure 19). The compounds featuring benzothiazole-2-thiol (Pt-23–Pt-25) exhibited red-shifted emission wavelengths compared to those incorporating benzoxazole-2-thiol (Pt-26–Pt-28) (Figure 20). The Pt-Pt distances were rigorously examined through theoretical studies, revealing that when the bridging ligand was substituted from benzothiazole to benzoxazole (as observed in the comparison between Pt-25 and Pt-28), the Pt-N bond distance shortened, leading to increased electron repulsion and a subsequent elongation of the Pt-Pt distance from 3.106 Å to 3.175 Å. As a result, Pt-25 (λmax = 722 nm) exhibited a narrower energy bandgap and a 26 nm red-shifted emission compared to Pt-28 (λmax = 696 nm).

3. Summary and Perspectives

Recent studies on double-decker-type dinuclear Pt(II) complexes suggest that shorter Pt-Pt distances generally lead to a decreased energy bandgap, thereby inducing longer wavelength emissions. In this type of compound, the new dσ* orbital, generated due to effective dz2 orbital overlap between the Pt atoms within the molecule, becomes the HOMO. As the Pt-Pt distance decreases, this orbital becomes increasingly unstable, leading to a reduction in the optical bandgap. These characteristics support the MMLCT transitions in dinuclear Pt(II) complexes. However, without effective dz2 orbital overlap, the MMLCT properties are significantly diminished, making it difficult to achieve deep-red or NIR emissions that require a narrow optical bandgap. There is no absolute proportionality between the Pt-Pt distance and the energy bandgap. Nevertheless, among structurally similar compounds with comparable cyclometalating and bridging ligands, a shorter relative Pt-Pt distance tends to decrease the energy bandgap. The Pt-Pt distance is primarily determined by the type of biting ligand and is also influenced by the electron density of the cyclometalating ligand. Furthermore, the structural flexibility of the ligands can affect the Pt-Pt distance. Consequently, strategies aimed at inducing longer wavelength emissions into the red and NIR regions should focus on minimizing the HOMO-LUMO energy gap. This approach should account for the energy levels established by the structure of the cyclometalating ligands and involve the sophisticated manipulation of Pt-Pt distances.
This review consolidates various factors affecting the optical bandgap and emission wavelengths of double-decker-type dinuclear Pt(II) complexes based on recent research findings. While no definitive methods have been established yet for adjusting the energy levels of these complexes, the latest research underlines the necessity of considering various factors and conditions to design appropriate molecular structures.
Looking towards the future, research into double-decker-type dinuclear Pt(II) complexes for red and NIR emission faces several challenges that must be addressed. One significant issue is the ease of oxidation from Pt(II) to Pt(III) due to the accessibility of the Pt(II) site. This oxidation process adversely affects luminescence efficiency, acting as a detrimental factor. Additionally, there is a need to contemplate strategies for enhancing the low luminescent efficiency in the long-wavelength region. Improving these properties can lead to devices that offer high performance, power efficiency, and a long lifetime when these complexes are applied as emissive materials in electronic devices.

Funding

This research was funded by Global—Learning & Academic research institution for masters, PhD students, and postdocs (LAMP) and the Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. RS2024-00444460).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Illustration of molecular stacking in Pt(II) complexes featuring square planar structures.
Figure 1. Illustration of molecular stacking in Pt(II) complexes featuring square planar structures.
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Figure 2. Simplified structural representation of different dinuclear Pt(II) complexes depending on the (a) absence or (b) presence of intramolecular Pt-Pt interactions.
Figure 2. Simplified structural representation of different dinuclear Pt(II) complexes depending on the (a) absence or (b) presence of intramolecular Pt-Pt interactions.
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Figure 3. Schematic representation of the molecular orbital illustrating dz2 orbital overlap in square planar Pt(II) complexes.
Figure 3. Schematic representation of the molecular orbital illustrating dz2 orbital overlap in square planar Pt(II) complexes.
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Figure 4. Molecular structures and photoluminescence spectra of NIR-phosphorescent π-extended platinum porphyrins in toluene. Adapted with permission from [79]; Copyright 2011 American Chemical Society.
Figure 4. Molecular structures and photoluminescence spectra of NIR-phosphorescent π-extended platinum porphyrins in toluene. Adapted with permission from [79]; Copyright 2011 American Chemical Society.
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Figure 5. (a) Structures of deep-red emissive dinuclear Pt(II) complexes and (b) PL spectra in diluted DCM solutions (1 × 10−5 M) containing long π-conjugated cyclometalating ligands. Image reproduced with permission from [80]; Copyright 2016 Elsevier.
Figure 5. (a) Structures of deep-red emissive dinuclear Pt(II) complexes and (b) PL spectra in diluted DCM solutions (1 × 10−5 M) containing long π-conjugated cyclometalating ligands. Image reproduced with permission from [80]; Copyright 2016 Elsevier.
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Figure 6. (a) Molecular structure and (b) photoluminescence spectra of red-phosphorescent π-extended Pt(II) complex, Pt-3. Image reproduced with permission from [81]; Copyright 2016 Elsevier.
Figure 6. (a) Molecular structure and (b) photoluminescence spectra of red-phosphorescent π-extended Pt(II) complex, Pt-3. Image reproduced with permission from [81]; Copyright 2016 Elsevier.
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Figure 7. Mercaptobenzothiazole-bridged dinuclear Pt(II) complexes, Pt-4−Pt-8. Redrawn from [82].
Figure 7. Mercaptobenzothiazole-bridged dinuclear Pt(II) complexes, Pt-4−Pt-8. Redrawn from [82].
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Crystals 15 00273 g008
Figure 8. (a) UV-vis absorption and (b) PL spectra in CH2Cl2 solutions and (c) single-crystal structures of dinuclear Pt(II) complexes, Pt-4−Pt-8. Image reproduced with permission from [82]; Copyright 2022 American Chemical Society.
Figure 8. (a) UV-vis absorption and (b) PL spectra in CH2Cl2 solutions and (c) single-crystal structures of dinuclear Pt(II) complexes, Pt-4−Pt-8. Image reproduced with permission from [82]; Copyright 2022 American Chemical Society.
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Crystals 15 00273 g009
Figure 9. Proposed orbital overlap in Pt-7. (a) Interaction of the filled dxz orbital (Pt) with the empty antibonding π*-orbitals (pyrazine) enabling π-backdonation. (b) Partial orbital overlap between the two dxz orbitals of Pt metals in the DFT calculation. Adapted with permission from [82]; Copyright 2022 American Chemical Society.
Figure 9. Proposed orbital overlap in Pt-7. (a) Interaction of the filled dxz orbital (Pt) with the empty antibonding π*-orbitals (pyrazine) enabling π-backdonation. (b) Partial orbital overlap between the two dxz orbitals of Pt metals in the DFT calculation. Adapted with permission from [82]; Copyright 2022 American Chemical Society.
Crystals 15 00273 g009
Crystals 15 00273 g010
Figure 10. Molecular structures of syn- and anti-isomers of double-decker-type dinuclear Pt(II) complexes. Redrawn from [27].
Figure 10. Molecular structures of syn- and anti-isomers of double-decker-type dinuclear Pt(II) complexes. Redrawn from [27].
Crystals 15 00273 g010
Crystals 15 00273 g011
Figure 11. Geometric isomers with different π-conjugation positions in C^N ligands. Image redrawn from [86].
Figure 11. Geometric isomers with different π-conjugation positions in C^N ligands. Image redrawn from [86].
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Crystals 15 00273 g012
Figure 12. Effect of substituents on the cyclometalating ligands to shorten the Pt-Pt distances. (a) Molecular structures and (b) Uv-vis absorption and PL spectra of Pt-15 and Pt-16. Image reproduced with permission from [10]; Copyright 2024 Elsevier.
Figure 12. Effect of substituents on the cyclometalating ligands to shorten the Pt-Pt distances. (a) Molecular structures and (b) Uv-vis absorption and PL spectra of Pt-15 and Pt-16. Image reproduced with permission from [10]; Copyright 2024 Elsevier.
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Crystals 15 00273 g013
Figure 13. (a) Structures of dinuclear Pt(II) complexes showing variations in ligand flexibility achieved by altering the ligand-connecting atoms and (b) single-crystal data of Pt-17 and Pt-18. (Hydrogen atoms are omitted for clarity). Image reproduced with permission from [87]; Copyright 2024 American Chemical Society.
Figure 13. (a) Structures of dinuclear Pt(II) complexes showing variations in ligand flexibility achieved by altering the ligand-connecting atoms and (b) single-crystal data of Pt-17 and Pt-18. (Hydrogen atoms are omitted for clarity). Image reproduced with permission from [87]; Copyright 2024 American Chemical Society.
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Crystals 15 00273 g014
Figure 14. (a) PL spectra of Pt-17 in (a) film state (2% in PMMA) and (b) solution state (in 1,2-dichloroethane) at room temperature. Adapted with permission from [87]; Copyright 2024 American Chemical Society.
Figure 14. (a) PL spectra of Pt-17 in (a) film state (2% in PMMA) and (b) solution state (in 1,2-dichloroethane) at room temperature. Adapted with permission from [87]; Copyright 2024 American Chemical Society.
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Crystals 15 00273 g015
Figure 15. Representative Pt–Pt distance of double-decker-type diplatinum (II) complexes depending on the bridging mode. Image reproduced with permission from [89]; Copyright 2023 WILEY.
Figure 15. Representative Pt–Pt distance of double-decker-type diplatinum (II) complexes depending on the bridging mode. Image reproduced with permission from [89]; Copyright 2023 WILEY.
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Crystals 15 00273 g016
Figure 16. Structural representation of bridging ligands in double-decker-type dinuclear Pt(II) complexes.
Figure 16. Structural representation of bridging ligands in double-decker-type dinuclear Pt(II) complexes.
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Crystals 15 00273 g017
Figure 17. Molecular structures of dinuclear Pt(II) complexes with different substituents on the bridging ligands. Image redrawn from [52].
Figure 17. Molecular structures of dinuclear Pt(II) complexes with different substituents on the bridging ligands. Image redrawn from [52].
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Crystals 15 00273 g018
Figure 18. Control of Pt-Pt separation by modulating the μ-pyrazolate bridging ligand and the resulting PL spectra of Pt-19, Pt-21, and Pt-22. Image reproduced with permission from [52]; Copyright 2005 American Chemical Society.
Figure 18. Control of Pt-Pt separation by modulating the μ-pyrazolate bridging ligand and the resulting PL spectra of Pt-19, Pt-21, and Pt-22. Image reproduced with permission from [52]; Copyright 2005 American Chemical Society.
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Crystals 15 00273 g019
Figure 19. Structures of dinuclear Pt(II) complexes with different bridging ligands containing atoms with different electronegativities. Image redrawn from [94].
Figure 19. Structures of dinuclear Pt(II) complexes with different bridging ligands containing atoms with different electronegativities. Image redrawn from [94].
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Crystals 15 00273 g020
Figure 20. UV-vis absorption and PL spectra of (a) Pt-23~Pt-25 and (b) Pt-26~Pt-28, and (c) comparison of the PL spectra of Pt-23−Pt-28. Image reproduced with permission from [94]; Copyright 2023 WILEY.
Figure 20. UV-vis absorption and PL spectra of (a) Pt-23~Pt-25 and (b) Pt-26~Pt-28, and (c) comparison of the PL spectra of Pt-23−Pt-28. Image reproduced with permission from [94]; Copyright 2023 WILEY.
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Park, H.J. Emission Wavelength Control via Molecular Structure Design of Dinuclear Pt(II) Complexes: Optimizing Optical Properties for Red- and Near-Infrared Emissions. Crystals 2025, 15, 273. https://doi.org/10.3390/cryst15030273

AMA Style

Park HJ. Emission Wavelength Control via Molecular Structure Design of Dinuclear Pt(II) Complexes: Optimizing Optical Properties for Red- and Near-Infrared Emissions. Crystals. 2025; 15(3):273. https://doi.org/10.3390/cryst15030273

Chicago/Turabian Style

Park, Hea Jung. 2025. "Emission Wavelength Control via Molecular Structure Design of Dinuclear Pt(II) Complexes: Optimizing Optical Properties for Red- and Near-Infrared Emissions" Crystals 15, no. 3: 273. https://doi.org/10.3390/cryst15030273

APA Style

Park, H. J. (2025). Emission Wavelength Control via Molecular Structure Design of Dinuclear Pt(II) Complexes: Optimizing Optical Properties for Red- and Near-Infrared Emissions. Crystals, 15(3), 273. https://doi.org/10.3390/cryst15030273

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