Synthesis and Luminescent Properties of cis Bis-N-Heterocyclic Carbene Platinum(II) Bis-Arylacetylide Complexes

Enhanced blue phosphorescence in platinum acetylide complexes via a secondary heavy metal and anion-controlled aggregation

Organoplatinum compounds represent a promising class of blue-phosphorescent molecules for electroluminescent color displays. Much recent work has focused on decreasing the nonradiative rate constant (k nr) to improve the photoluminescence quantum yield (Φ PL) of these compounds, but in most cases small radiative rate constants (k r) lead to long excited-state lifetimes (τ) poorly suited for electroluminescence applications. In this work, we present an approach to increase k r and Φ PL in blue-phosphorescent platinum acetylide complexes with the general formula cis-[Pt(CN-R)2(C[triple bond, length as m-dash]C-2-py)2] (CN-R is an alkyl isocyanide and C[triple bond, length as m-dash]C-2-py is 2-pyridylacetylide). This method incorporates secondary heavy metals, Cu(i) or Ag(i), bound by the pyridyl moieties. We observe the formation of dimer complexes in the solid state due to noncovalent interactions between the secondary metal and the acetylide ligands, especially strong in the case of Cu(i). Incorporation of Cu(i) also erodes the desired blue-phosphorescence by introducing a low-lying metal-to-ligand charge transfer (3MLCT) state that dominates the observed phosphorescence. In the complexes bound to Ag(i), we find that phosphorescence profile is strongly dependent on the counteranion, which we propose is caused by different degrees of aggregation. With this insight, we show that coordination of AgBArF 4 (BArF 4 - = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), with a large noncoordinating counteranion, inhibits aggregation and results in a 4-8× increase in k r and a 5-10× increase in Φ PL while preserving a pure blue phosphorescence profile.

Luminescent Platinum Complexes with π-Extended Aryl Acetylide Ligands Supported by Isocyanides or Acyclic Diaminocarbenes

In this work, we present a series of luminescent platinum acetylide complexes with acetylides that are electronically modified and/or π-extended. Six isocyanide-supported complexes with the general formula cis-[Pt(CNAr)2(C≡CR)2] and six acyclic diaminocarbene (ADC) complexes of the form trans-[Pt(ADC)2(C≡CR)2], all using the same five acetylide ligands, are described. The compounds are characterized by multinuclear NMR, FT-IR, and single-crystal X-ray diffraction. In most cases, the phosphorescence arises from an acetylide-centered 3(π → π*) excited state, although in one of the isocyanide compounds there is evidence for a charge-transfer excited state. The photoluminescence wavelength depends strongly on the substitution pattern and extent of the π conjugation on the acetylide, with maxima spanning the range of ca. 460-540 nm. Most photoluminescence lifetimes are long, beyond 50 μs, and quantum yields are low to moderate, 0.043-0.27. The photoluminescence quantum yields and lifetimes in these compounds do not systematically improve in the ADC complexes compared to the isocyanide versions, suggesting the neutral ligand σ-donor character does not play a large role in the excited-state dynamics when the triplet excited state is delocalized over a large π system.

Effect of bromine substitution on blue phosphorescent trans-(N-heterocyclic carbene)Pt(II) acetylide complexes

N-heterocyclic carbene complexes of the type trans-(NHC)2PtII(CC-Ar)2 (where Ar = phenyl or substituted phenyl) are of interest as violet and blue phosphors. These complexes emit efficient phosphorescence in solution and in the solid state, and they have been applied as phosphors in organic light emitting diodes. This study explores the effect of bromine substitution on the trans-(NHC)2PtII(CC-Ar)2 chromophore through photophysical studies of a pair of complexes in which the phenyl groups feature either 3,5-dibromo- or 4-monobromo-substituents (IPt-DB and IPt-MB, respectively). The Br atoms were introduced as heavy atom(s) with the aim to enhance spin-orbit coupling and increase the radiative and non-radiative decay rates of the phosphorescent triplet state. Both IPt-MB and IPt-DB exhibit sky-blue phosphorescence in solution and in PMMA matrix. Interestingly, the emission quantum yield and lifetime of IPt-MB are substantially lower compared to IPt-DB in solution. This effect is attributed to a substantially larger non-radiative decay rate in the mono-bromo complex. Analysis of the photophysical data, combined with DFT and TD-DFT calculations, suggest that the difference in photophysical properties of the two complexes is related to the position of the Br-substituents on the phenyl acetylide rings. In short, in IPt-MB, the Br-substituents are located para-to the Pt-CC-unit, and this gives rise to stronger electron-vibrational coupling in the excited state, enhancing the rate of non-radiative decay.

Efficient blue-phosphorescent trans-bis(acyclic diaminocarbene) platinum(ii) acetylide complexes

The lack of efficient and robust deep-blue phosphorescent metal complexes remains a significant challenge in the context of electroluminescent color displays. The emissive triplet states of blue phosphors are deactivated by low-lying metal-centered (³MC) states, which can be ameliorated by increasing the σ-donating ability of the supporting ligands. Here we unveil a synthetic strategy to access blue-phosphorescent complexes with two supporting acyclic diaminocarbenes (ADCs), known to be even stronger σ-donors than N-heterocyclic carbenes (NHCs). This new class of platinum complexes has excellent photoluminescence quantum yields, with four of six complexes affording deep-blue emission. Experimental and computational analyses are consistent with a pronounced destabilization of the ³MC states by the ADCs.

Platinum(II)-Substituted Phenylacetylide Complexes Supported by Acyclic Diaminocarbene Ligands

We introduce phosphorescent platinum aryl acetylide complexes supported by tert-butyl-isocyanide and strongly σ-donating acyclic diaminocarbene (ADC) ligands. The precursor complexes cis-[Pt(CNtBu)₂(C≡CAr)₂] (4a-4f) are treated with diethylamine, which undergoes nucleophilic addition with one of the isocyanides to form the cis-[Pt(CNtBu)(ADC)(C≡CAr)₂] complexes (5a-5f). The new compounds incorporate either electron-donating groups (4-OMe and 4-NMe₂) or electron-withdrawing groups [3,5-(OMe)₂, 3,5-(CF₃)₂, 4-CN, and 4-NO₂] on the aryl acetylide. Experimental HOMO-LUMO gaps, estimated from cyclic voltammetry, span the range of 2.68-3.61 eV and are in most cases smaller than the unsubstituted parent complex, as corroborated by DFT. In the ADC complexes, peak photoluminescence wavelengths span the range of 428 nm (2a, unsubstituted phenylacetylide) to 525 nm (5f, 4-NO₂-substituted), with the substituents inducing a red shift in all cases. The phosphorescence E_0,0 values and electrochemical HOMO-LUMO gaps are loosely correlated, showing that both can be reduced by either electron-donating or electron-withdrawing substituents on the aryl acetylides. The photoluminescence quantum yields in the ADC complexes are between 0.044 and 0.31 and the lifetimes are between 4.8 and 14 μs, a factor of 1.8-10× higher (for Φ_PL) and 1.2-3.6× longer (for τ) than the respective isocyanide precursor (Φ_PL = 0.014-0.12, τ = 2.8-8.2 μs).

The Search for Efficient True Blue and Deep Blue Emitters: An Overview of Platinum Carbene Acetylide Complexes

Despite significant strides achieved in organic light emitting diode (OLED) based display technologies during the last decade, the search for highly stable and efficient true blue/deep blue emitters continues to remain elusive. During the past decade, emitters with the basic molecular scaffold consisting of Pt(II) acetylides and N-heterocyclic carbene (NHC) ligands have opened interesting opportunities to tune the emission properties with desired chromaticity in the blue and deep blue region. With an aim to achieve the desired CIE coordinates along with low device roll-off efficiencies and satisfactory color purity, a number of variations on the basic molecular fragment have been made. A number of NHC Pt(II) alkyne complexes bearing monodentate, bidentate and tridentate ligands have been synthesized and their photophysical investigations have been reported. Although NHC Pt(II) alkyne complexes have been explored in other areas of applications, much of the investigations have been primarily focused for their interesting emission properties appearing particularly in the shorter wavelength (450-495 nm) part of the electromagnetic spectrum for organic light emitting diode (OLED) devices. In this review, we provide an overview of the investigated NHC Pt(II) acetylide complexes by detailing their synthesis, luminescence properties, performance in devices and future perspectives.

Luminescent Platinum(II) Complexes with Bidentate Diacetylide Ligands: Structures, Photophysical Properties and Application Studies

A series of platinum(II) complexes supported by terphenyl diacetylide as well as diimine or bis-N-heterocyclic carbene (NHC) ligands have been prepared. The diacetylide ligands adopt a cis coordination mode featuring non-planar terphenyl moieties as revealed by X-ray crystallographic analyses. The electrochemical, photophysical and photochemical properties of these platinum(II) complexes have been investigated. These platinum(II) diimine complexes show broad emission with peak maxima from 566 nm to 706 nm, with two of them having emission quantum yields >60% and lifetimes <2 μs in solutions at room temperature, whereas the platinum(II) diacetylide complexes having bis-N-heterocyclic carbene instead of diimine ligand display photoluminescence with quantum yields of up to 28% in solutions and excited state lifetimes of up to 62 μs at room temperature. Application studies revealed that one of the complexes can catalyze photoinduced aerobic dehydrogenation of alcohols and alkenes, and a relatively non-toxic water-soluble Pt(II) complex displays anti-angiogenic activity.

Efficient Deep Blue Platinum Acetylide Phosphors with Acyclic Diaminocarbene Ligands

Here we report five blue-phosphorescent platinum bis-phenylacetylide complexes with an investigation of their photophysical and electrochemical attributes. Three of the complexes (1-3) are of the general formula cis-Pt(CNR)₂ (C≡CPh)₂ , in which CNR is a variably substituted isocyanide and C≡CPh is phenylacetylide. These isocyanide complexes serve as precursors for complexes of the general formula cis-Pt(CNR)(ADC)(C≡CPh)₂ (4 and 5), in which ADC is an acyclic diaminocarbene installed by amine nucleophilic addition to one of the isocyanides. All of the complexes exhibit deep blue phosphorescence with λ_max ∼430 nm in poly(methyl methacrylate) (PMMA) thin films. Whereas isocyanide complexes 1-3 exhibit modest photoluminescence quantum yields (Φ_PL ), incorporation of one acyclic diaminocarbene ligand results in a three-fold to 16-fold increase in Φ_PL while still maintaining an identical deep blue color profile.

Stereochemical Effects on Platinum Acetylide Two-Photon Chromophores

A series of cis-platinum(II) acetylide complexes containing two-photon-absorbing chromophores have been synthesized and characterized to explore the effects of stereochemistry on the nonlinear absorption properties. The molecules feature 4-(phenylethynyl)phenylethynylene (PE2), diphenylaminofluorene (DPAF), and benzothiazolylfluorene (BTF) ligands. The photophysical properties were investigated under one- and two-photon conditions and compared to the known trans analogues via UV-visible absorption, photoluminescence, femtosecond and nanosecond transient absorption (TA), nanosecond z-scan, and femtosecond two-photon absorption (2PA). The bent cis complexes exhibit blue shifts in the absorption, emission, femtosecond, and nanosecond TA spectra along with lower molar extinction coefficients and lower phosphorescence yields relative to the trans complexes suggesting less efficient Pt-induced spin-orbit coupling and intersystem crossing in the cis configuration. The cis chromophores are noncentrosymmetric and therefore show dipolar behavior with a pronounced 2PA in the 0-0 transition of the S₀ → S₁ band, while the trans complexes show quadrupolar behavior with a forbidden 0-0 transition. In the S₀ → S_n region, both cis and trans complexes show intense two-photon-absorption bands (up to 3700 GM by the peak cross section for cis-BTF) which contain a significant contribution from the excited state absorption (S₁ → S_n). All six complexes exhibit comparable nonlinear absorption response with a significant contribution from triplet-triplet absorption that slightly favors trans complexes but is more strongly dependent upon the structure of the π-conjugated chromophore.

Rise of Conjugated Poly-ynes and Poly(Metalla-ynes): From Design Through Synthesis to Structure-Property Relationships and Applications

Conjugated poly-ynes and poly(metalla-ynes) constitute an important class of new materials with potential application in various domains of science. The key factors responsible for the diverse usage of these materials is their intriguing and tunable chemical and photophysical properties. This review highlights fascinating advances made in the field of conjugated organic poly-ynes and poly(metalla-ynes) incorporating group 4-11 metals. This includes several important aspects of conjugated poly-ynes viz. synthetic protocols, bonding, electronic structure, nature of luminescence, structure-property relationships, diverse applications, and concluding remarks. Furthermore, we delineated the future directions and challenges in this particular area of research.

4,5-Substituted C^C* cyclometalated thiazol-2-ylidene platinum(ii) complexes - synthesis and photophysical properties

We report the synthesis of seven novel backbone functionalized N-phenyl-1,3-thiazol-2-ylidene platinum(ii) complexes and their photophysical properties. Electronically diverse N-phenyl-1,3-thiazol-2-thiones were prepared by a reaction of aniline with carbon disulfide and different α-haloketone compounds. Oxidative desulfuration and salt metathesis yielded the desired NHC-precursors with hexafluorophosphate counterions. In addition, a new route for the synthesis of N-phenyl-1,3-benzo[d]thiazole tetrafluoroborate via N-arylation using hypervalent iodine species is presented. All complexes were prepared from the corresponding NHC precursor in a one-pot process using silver(i)oxide, transmetalation to platinum and reaction with the β-diketone acetylacetone under basic conditions. These complexes exhibit strong phosphorescence with quantum yields up to 72% in 2 wt% PMMA films with decay lifetimes of 8.8-12.3 μs. The influence of methyl- and phenyl-groups, and an ester-substituent at the 4- and/or 5-position of the 1,3-thiazole moiety, as well as the N-phenyl-1,3-benzo[d]thiazole-derived motif is discussed. The 4,5-unsubstituted-N-phenyl-1,3-thiazol-2-ylidene platinum(ii) acetylacetonato complex served as a reference in this study to evaluate the electronic effects originating from the backbone substitution. All complexes emit in a narrow range of the bluish-green spectrum of the visible light (510 ± 10 nm).

U- to Z-shape isomerization in a Pt2Ag2 framework containing pyridyl-NHC ligands

A highly twisted U-shaped Pt₂Ag₂ complex bearing pyridyl–NHC ligands showed isomerization to Z-shaped conformations via an intermolecular process. ¹H NMR experiments revealed that the isomerization reached U : Z = 2 : 1 ratio in a thermodynamic equilibrium state.

Heteroleptic Cycloplatinated N-Heterocyclic Carbene Complexes: A New Approach to Highly Efficient Blue-Light Emitters

New heteroleptic compounds of platinum(II)-containing cyclometalated N-heterocyclic carbenes, [PtCl(R-C^C*)(PPh₃)] [R-CH^C*-κC* = 3-methyl-1-(naphthalen-2-yl)-1H-imidazol-2-ylidene (R-C = Naph; 1A), 1-[4-(ethoxycarbonyl)phenyl]-3-methyl-1H-imidazol-2-ylidene (R = CO₂Et; 1B), and [Pt(R-C^C*)(py)(PPh₃)]PF₆ (py = pyridine; R-C = Naph, 2A; R = CO₂Et, 2B], have been prepared and fully characterized. All of them were obtained as the trans-(C*,PPh₃) isomer in high yields. The selectivity of their synthesis has been explained in terms of the degree of transphobia (T) of pairs of ligands in trans positions. X-ray diffraction studies on both 2A and 2B revealed that only in 2A, containing a C^C* with a more extended π system, do the molecules assemble themselves into head-to-tail pairs through intermolecular π···π contacts. The photophysical properties of 2A and 2B and those of the related compounds [Pt(NC-C^C*)(PPh₃)L]PF₆ [NC-CH^C*-κC* = 1-(4-cyanophenyl)-3-methyl-1H-imidazol-2-ylidene; L = pyridine (py; 2C), 2,6-dimethylphenylisocyanide (CNXyl; 3C), and 2-mercapto-1-methylimidazole (MMI; 4C)] have been examined to analyze the influence of the R substituent on R-C^C* (R-C = Naph; R = CO₂Et, CN) and that of the ancillary ligands (L) on them. Experimental data and time-dependent density functional theory calculations showed the similarity of the electronic features associated with R-C^C* (R = CN, CO₂Et) and their difference with respect to R-C^C* (R-C = Naph). All of the compounds are very efficient blue emitters in poly(methyl methacrylate) films under an argon atmosphere, with QY values ranging from 68% (2B) to 93% (2C). In the solid state, the color of the emission changes to yellowish-orange for compounds 2A (λ_max = 600 nm) and 3C (λ_max = 590 nm) because of the formation of aggregates through intermolecular π···π interactions. 2C and 3C were chosen to fabricate fully solution-processed electroluminescent devices with blue-light (2C), yellow-orange-light (3C), and white-light (mixtures of 2C and 3C) emission from neat films of the compounds as emitting layers.

Photophysical Enhancement of Triplet Emitters by Coordination-Driven Self-Assembly

The quantum yields of organic fluorophores used as donors in coordination-driven self-assembly often suffer from the heavy atom effect of nearby metal sites. Here, the role of intersystem crossing from a deactivating process to one that delivers emissive triplet states was reversed. A phosphorescent trans bis-N-heterocyclic carbene platinum(II) compound, Pt(dhim)₂ (C≡C-4-py)₂ (D1; dhim=1,3-dihexyl-2-H-imidazol-2-ylidene), was used along with other linear donors 4,4'-bipyridine (D2) and 1,4-bis(4-pyridyl ethynyl)benzene (D3) in self-assembly reactions with Pt(dtbpy)X₂ acceptors (dtbpy=4,4'-di-tert-butyl-2,2'-bipyridine) to afford three metallacycles. Photophysical investigations revealed that, although the building blocks used to construct M1 have relatively low quantum yields (Φ=1.2 and <1 % for D1 and 2, respectively), the metallacycle has a quantum yield of 14 %. This increase reflects a change in radiative rate constant from 3.6×10⁴  s^-1 for D1 to 2.1×10⁵  s^-1 for M1.

Changing the Emission Properties of Phosphorescent C^C*-Cyclometalated Thiazol-2-ylidene Platinum(II) Complexes by Variation of the β-Diketonate Ligands

Cyclometalated thiazol-2-ylidene platinum(II) complexes based on the N-phenyl-4,5-dimethyl-1,3-thiazol-2-ylidene N-heterocyclic carbene (NHC) ligand and seven different β-diketonate ligands have been synthesised and investigated for their structural and photophysical properties. The complexes were synthesised in a one-pot procedure starting with the in situ formation of the corresponding silver(I) carbene and transmetalation to platinum, followed by the reaction with the respective β-diketonate under basic conditions. All the compounds were fully characterised by standard techniques, including ¹⁹⁵ Pt NMR spectroscopy. Three solid-state structures revealed quite different aggregation behaviour depending on the β-diketonate architecture. The reported complexes showed strong phosphorescence at room temperature in amorphous poly(methyl methacrylate) films. The emission wavelengths (ca. 510 nm) were found to be independent of the β-diketonate ligand, but the electronically diverse β-diketonates strongly influence the observed quantum yields (QY) and decay lifetimes. The results of theoretical studies employing density functional theory (DFT) methods support the conclusion of a metal-to-ligand charge transfer (³ MLCT) as the main emission process, in accordance with the reported photophysical properties. Standard organic light-emitting diodes (OLEDs) prepared with unoptimised matrix materials using one of the complexes showed values of 12.3 % external quantum yield, 24.0 lm W^-1 luminous efficacy and 37.8 cd A^-1 current efficiency at 300 cd m^-2 .

Triboluminescence and Metal Phosphor for Organic Light-Emitting Diodes: Functional Pt(II) Complexes with Both 2-Pyridylimidazol-2-ylidene and Bipyrazolate Chelates

We report the utilization of both pyrid-2-yl-imidazolylidene and dianionic bipz chelates as constituents in syntheses of a new series of charge-neutral Pt(II) complexes 1-4, among which complex 4 revealed remarkable triboluminescence, i.e., generation of photoemission upon grinding or cracking of the solid sample. The triboluminescence is found to be sensitive to the subtle changes of the associated substituents of pyrid-2-yl-imidazolylidene chelate, as verified by the disappearance of the triboluminescence for complexes 1-3. Alternatively, the well-ordered solid packing of 3, as indicated by the grazing incidence X-ray scattering experiment, serves as an ideal emitter for the fabrication of highly efficient OLEDs, rendering high external quantum efficienciy (25.9%) and luminesce efficiency (90 cd A^-1) at the practical brightness of 100 cd m^-2. The rather low roll-off in efficiency (24.4%, 85 cd A^-1 at high brightness of 1000 cd m^-2) is attributed to the short excited-state lifetime of 3 (∼800 ns) in the solid state, which in turn is associated with the MMLCT transition character.

Pincer-CNC mononuclear, dinuclear and heterodinuclear Au(III) and Pt(II) complexes supported by mono- and poly-N-heterocyclic carbenes: synthesis and photophysical properties

A family of cyclometallated Au(iii) and Pt(ii) complexes containing a CNC-pincer ligand (CNC = 2,6-diphenylpyridine) supported by pyrene-based mono- or bis-NHC ligands have been synthesized and characterized, together with the preparation of a Pt-Au hetero-dimetallic complex based on a Y-shaped tris-NHC ligand. The photophysical properties of all the new species and of two related Ru(ii)-arene complexes were studied and compared. Whereas the pyrene-based complexes only exhibit emission in solution, those containing the Y-shaped tris-NHC ligand are only luminescent when dispersed in poly(methyl methacrylate) (PMMA). In particular, the pyrene-based complexes were found to be emissive in the range of 373-440 nm, with quantum yields ranging from 3.1 to 6.3%, and their emission spectra were found to be almost superimposable, pointing to the fluorescent pyrene-centered nature of the emission. This observation suggests that the emission properties of the pyrene fragment may be combined with some of the numerous applications of NHCs as supporting ligands allowing, for instance, the design of biological luminescent agents.

Intense blue emission and a reversible hypsochromic shift of luminescence caused by grinding based on silver(i) complexes

Highly blue-emissive silver(i) complexes bearing N-heterocyclic carbene and diphenylphosphinobenzene were newly synthesized. The reversible hypsochromic shift of the emission by the grinding process was observed.

Pincer-Type Platinum(II) Complexes Containing N-Heterocyclic Carbene (NHC) Ligand: Structures, Photophysical and Anion-Binding Properties, and Anticancer Activities

Two classes of pincer-type Pt(II) complexes containing tridentate N-donor ligands (1-8) or C-deprotonated N^C^N ligands derived from 1,3-di(2-pyridyl)benzene (10-13) and auxiliary N-heterocyclic carbene (NHC) ligand were synthesized. [Pt(trpy)(NHC)](2+) complexes 1-5 display green phosphorescence in CH2 Cl2 (Φ: 1.1-5.3 %; τ: 0.3-1.0 μs) at room temperature. Moderate-to-intense emissions are observed for 1-7 in glassy solutions at 77 K and for 1-6 in the solid state. The [Pt(N^C^N)(NHC)](+) complexes 10-13 display strong green phosphorescence with quantum yields up to 65 % in CHCl3 . The reactions of 1 with a wide variety of anions were examined in various solvents. The tridentate N-donor ligand of 1 undergoes displacement reaction with CN(-) in protic solvents. Similar displacement of the N^C^N ligand by CN(-) has been observed for 10, leading to a luminescence "switch-off" response. The water-soluble 7 containing anthracenyl-functionalized NHC ligand acts as a light "switch-on" sensor for the detection of CN(-) ion with high selectivity. The in vitro cytotoxicity of the Pt(II) complexes towards HeLa cells has been evaluated. Complex 12 showed high cytotoxicity with IC50 value of 0.46 μM, whereas 1-4 and 6-8 are less cytotoxic. The cellular localization of the strongly luminescent complex 12 traced by using emission microscopy revealed that it mainly localizes in the cytoplasmic structures rather than in the nucleus. This complex can induce mitochondria dysfunction and subsequent cell death.

Dual ligand-based fluorescence and phosphorescence emission at room temperature from platinum thioxanthonyl complexes

Platinum complexes with σ-bonded thioxanthonyl (Tx) ligands exhibit, on irradiation into the Tx π→π* band, dual Tx-based fluorescence and phosphorescence emission with phosphorescence quantum yields of up to 19% in fluid solution at room temperature.

N-heterocyclic carbene metal complexes: photoluminescence and applications

This review covers the advances made in the synthesis and applications of luminescent transition metal complexes containing N-heterocyclic carbene (NHC) ligands.

Photophysical properties of trans-platinum acetylide complexes featuring N-heterocyclic carbene ligands

Platinum acetylide complexes featuring NHC-carbene ligands show enhanced photophysical properties relative to complexes with phosphine ligands.

Spin–orbit effects in square-planar Pt(ii) complexes with bidentate and terdentate ligands: theoretical absorption/emission spectroscopy

Theoretical “spin-free” (in black) and “spin–orbit” (in red) absorption spectra of [Pt(tpy)Cl]⁺ and electronic densities characterizing the main intense bands MLCT/XLCT at 384 nm, and MLCT/LC at 326 nm and 250 nm (the experimental spectrum is shown in the inset).

Emissive osmium(II) complexes supported by N-heterocyclic carbene-based C^C^C-pincer ligands and aromatic diimines

Osmium(II) complexes containing N-heterocyclic carbene (NHC)-based pincer ligand 1,3-bis(1-methylimidazolin-2-ylidene)phenyl anion (C(1)^C^C(1)) or 1,3-bis(3-methylbenzimidazolin-2-ylidene)phenyl anion (C(2)^C^C(2)) and aromatic diimine (2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), or 4,4'-diphenyl-2,2'-bipyridine (Ph(2)bpy)) in the form of [Os(C^C^C)(N^N)(CO)](+) have been prepared. Crystal structures for these complexes show that the Os-C(NHC) bonds are essentially single (Os-C(NHC) distances = 2.079(5)-2.103(7) Å). Spectroscopic comparisons and time-dependent density functional theory (TD-DFT) calculations suggest that the lowest-energy electronic transition associated with these complexes (λ(max) = 493-536 nm, ε(max) = (5-10) × 10(3) dm(3) mol(-1) cm(-1), solvent = CH(3)CN) originate from a d(π)(Os(II)) → π*(N^N) metal-to-ligand charge transfer transition, where the d(π)(Os(II)) and π*(N^N) levels contain significant contribution from the C^C^C ligands. All these complexes are emissive in the red-spectral region (674-731 nm) with quantum yields of 10(-4)-10(-2) and emission lifetimes of around 1-6 μs. Transient absorption spectroscopy and spectroelectrochemical measurements have also been used to probe the nature of the emissive excited-states. Overall, this joint experimental and theoretical investigation reveals that the C^C^C ligands can be used to modulate the photophysical properties of a [Os(N^N)] core via the formation of the hybrid [Os + C^C^C] frontier orbitals.