Luminescent Platinum Compounds: From Molecules to OLEDs

Abstract

Around 30 years ago, much of the research into platinum coordination chemistry was being driven either by research into one-dimensional, electrically conducting molecular materials exploiting the stacking interactions of planar complexes, or by the unprecedented success of cis-Pt(NH₃)₂Cl₂ (cisplatin) as an anticancer agent. At that time, a number of simple platinum(II) compounds were known to be photoluminescent at low temperature or in the solid state, but almost none in fluid solution at room temperature. Since that time, several families of complexes have been discovered that are brightly luminescent, and a number of investigations have shed light on the factors that govern the luminescence efficiencies of Pt(II) complexes. Over the past decade, such studies have been spurred on by the potential application of triplet-emitting metal complexes as phosphors in organic light-emitting devices (OLEDs), where their ability to trap otherwise wasted triplet states can lead to large gains in efficiency. In this contribution, we take a chemist's perspective of the field, overviewing in the first instance the factors that need to be taken into account in the rational design of highly luminescent platinum(II) complexes, and the background to their use in OLEDs. We then consider in more detail the properties of some individual classes, highlighting work from the past 3 years, and including selected examples of their utility in OLEDs and other applications.

1 Fluorescence and Phosphorescence: Singlets and Triplets

It was Alexandre-Edmond Becquerel, in his 1867 treatise La Lumière, ses causes et ses effets, who first put forward a systematic distinction between fluorescence and phosphorescence. Becquerel designed a phosphoroscope that allowed precise time intervals to elapse between the exposure of a material to light and the observation of the light emitted. He defined fluorescence as emission of light that is immediately extinguished upon removal of the light source, whilst phosphorescence persists for some time after exposure.

The modern and more rigorous distinction is based in quantum theory. Fluorescence is the emission of light that occurs as the result of a spin-allowed electronic transition (ΔS = 0), for example, from the first excited singlet state of an aromatic molecule to the ground state, S₁ → S₀ [1]. It is normally characterised by a high radiative rate constant ${k_{\rm{r}}^{\rm{S}}}$ of the order of 10⁸–10⁹ s^–1, leading to short emission lifetimes in the nanosecond range. Phosphorescence arises from spin-forbidden transitions (ΔS ≠ 0), for example, from the first excited triplet state to the ground state, T₁ → S₀. In this case, radiative rate constants ${k_{\rm{r}}^{\rm{T}} }$ are frequently small, around 1 s⁻¹ for molecules such as anthracene, leading to natural lifetimes of the order of 1 s [2]. In fact, the phosphorescent process is so slow that faster processes of non-radiative decay of the triplet state normally predominate for such molecules in solution at room temperature, so that phosphorescence is not observed. Only by cooling the sample down to low temperatures and rigidifying it to inhibit such processes is the phosphorescent light observable. The key radiative and non-radiative processes involved are typically represented by a Jabłonski diagram; an example for an organic compound is given in Fig. 1.

Fig. 1

A simplified Jabłonski diagram for a conjugated organic molecule such as anthracene, illustrating the rate constants for key processes as defined in the text. ‘IC’ represents internal conversion (an isoenergetic process) followed by vibrational relaxation; similarly ‘ISC’ represents intersystem crossing followed by vibrational relaxation

In the case of photoluminescence, where the molecule is excited by light, the proportion of triplet and singlet states formed depends upon the relative magnitudes of ${k_{\rm{r}}^{\rm{S}} }$ and k _ISC, where k _ISC is the rate constant for intersystem crossing from S₁ to T₁. For simple conjugated molecules like naphthalene, anthracene and pyrene, these two parameters have similar values to one another, leading to fluorescence quantum yields of the order of 0.5. In contrast, in cases such as benzophenone, where the difference in energy between S₁ and T₁ is very small, k _ISC may be very high such that no significant fluorescence is observed, and the efficiency of triplet formation approaches unity.

Irrespective of the triplet yield, once formed, the radiative rate constant of the triplet state is normally low. If a heavy atom – such as a third row transition metal ion – is brought close to the π-system of the molecule, in such a way that there is significant mixing of the orbitals, then ${k_{\rm{r}}^{\rm{T}} }$ can be greatly increased, sometimes by up to 10⁶ or more, due to the large spin–orbit coupling associated with the heavy atom which scales with Z ⁴. Radiative emission can now compete with deactivating processes allowing phosphorescence to be observed at room temperature.

2 Electroluminescence and OLEDs

Organic light-emitting devices (OLEDs) are based on electroluminescence. The energy required to raise the emitting molecule to the excited state is supplied electrically, rather than by the absorption of higher-energy light as it is in photoluminescence. Briefly, a thin layer of the emissive material is sandwiched between two electrodes [3]. Upon application of the electric field, the material is reduced at the cathode and oxidised at the anode to give electron and hole carriers, respectively, which migrate under the applied field towards the other electrode. When two such charge carriers of opposite sign meet, they combine to produce one molecule in the ground state and one in the excited state (Fig. 2). In contrast to photoluminescence, both singlet and triplet excited states (also known as excitons) are now formed directly, and their proportion is governed primarily by the statistics of charge recombination, leading to a theoretical 1:3 (S:T) ratio [4]. In a purely organic light-emitting device, emission from the triplet states is forbidden, so the efficiency is capped at 25%. This represents a huge wastage of energy, which simply goes into heating up the device.

Fig. 2

Schematic representation of electroluminescence in organic materials

As noted in Sect. 1, the introduction of a heavy metal ion onto a conjugated organic molecule can greatly accelerate the S ← T process through the influence of spin–orbit coupling. Thus, by incorporating such phosphorescent materials into the emissive layer of the device, emission from the triplet excitons can be promoted, and quantum efficiencies of 100% potentially achieved.

The first metal-based phosphorescent dopant to be investigated as such a ‘triplet-harvesting’ dopant in an OLED was platinum(II) octaethylporphyrin, 1 [5]. Upon doping this compound into the widely-used EL emissive material Alq₃ {tris(8-quinolinato)aluminium, 2}, 90% of the energy is transferred from the host to the porphyrin, and a device of internal electroluminescence efficiency of 23% (external EL = 4%) obtained [6]. Given that the triplets are now clearly emitting too, why isn't the device efficiency higher? Part of the reason is the long phosphorescence lifetime of 1 (∼80 μs in solution), which results in severe triplet-triplet annihilation, particularly at high current (Eq. 1); in other words, the triplet excited state is so long-lived that it survives long enough to encounter other triplet excitons, and half the energy is lost non-radiatively. Clearly, there is a need for new complexes that emit efficiently but with shorter radiative lifetimes. Ways of achieving this are addressed in the following section.

$${\rm{4(}}^{\rm{3}} {\rm{M^{\ast} + }}^{\rm{3}} {\rm{M^{\ast}) }} \to {\rm{ }}^{\rm{1}} {\rm{M^{\ast} + }}{\rm{3}}^{\rm{3}} {\rm{M^{\ast} + 4M}}{\rm{.}}$$

(1)

3 Designing Highly Luminescent Platinum Complexes

In common with fluorescent molecules, the efficiency of luminescence of metal complexes from triplet excited states, $\eta _{\rm{r}}^{\rm{T}} ,$ will be favoured by high radiative rate constants ${k_{\rm{r}}^{\rm{T}} }$ and low rate constants for non-radiative decay pathways $\sum {k_{\rm{r}}^{\rm{T}} }$:

$$\eta _{\rm{r}}^{\rm{T}} = k_{\rm{r}}^{\rm{T}} /\left\{ {k_{\rm{r}}^{\rm{T}} + \sum {k_{{\rm{nr}}}^{\rm{T}} } } \right\}.$$

(2)

In photoluminescence, the observed triplet luminescence quantum yield, ${\Phi _{{\rm{lum}}}^{\rm{T}} ,}$ also depends on the efficiency of formation of the triplet state upon absorption of light, in turn determined by the magnitude of the rate constant of intersystem crossing, k _ISC (Eq. 3), where ${k_{\rm{r}}^{\rm{S}} }$ and ${\sum {k_{{\rm{nr}}}^{\rm{S}} } }$ are the radiative and non-radiative decay rate constants of the singlet state:

$$\Phi _{{\rm{lum}}}^{\rm{T}} {\rm{ \,=\, }}\eta _{\rm{r}}^{\rm T} \cdot {\rm{ }}k_{{\rm{ISC}}} /\left\{ {k_{\rm{r}}^{\rm{S}} + \sum {k_{{\rm{nr}}}^{\rm{S}} } } \right\}.$$

(3)

The high spin–orbit coupling constant of the platinum nucleus (χ = 4,481 cm⁻¹) should facilitate both the S → T intersystem crossing process and the T → S radiative decay. However, the extent to which it does so in a complex will depend upon the contribution of metal atomic orbitals to the excited state. In many simple Pt(II) complexes with relatively small ligands, the metal's involvement is such that triplet state formation is very fast, of the order of 10¹² s⁻¹ [7]. Since this greatly exceeds typical singlet radiative rate constants of aromatic ligands, emission is then observed exclusively from the triplet states. On the other hand, if the excited state is localised on a remote part of a more extended ligand, well removed from the metal centre, or if the pertinent orbitals are orthogonal and hence poorly mixed, ligand-based singlet fluorescence may be observed [8, 9].

Similarly, the radiative rate constant ${k_{\rm{r}}^{\rm{T}} }$ is likely to be larger for excited states in which the metal plays a significant role; e.g. MLCT states, or LC states that are significantly perturbed by the metal [10].

Equally important is the need to minimise the rate of non-radiative decay, Σk _nr. In the case of square planar platinum(II) complexes, this is often determined primarily by the energy of metal-centred (d–d) states, a point that can be readily understood in terms of the ligand-field splitting diagram of square planar d ⁸ complexes (Fig. 3a) [11]. The unoccupied d _{x 2–y2} orbital is strongly antibonding. Population of this orbital will therefore be accompanied by elongation of Pt–L bonds and severe distortion, which promotes non-radiative decay to the ground state due to the isoenergetic crossing point lying at accessible energies (Fig. 3b). Of course, in many platinum(II) complexes, LC and MLCT states may lie lower in energy than these d–d states, but the latter will still exert a deleterious influence if they are thermally accessible, i.e. if ΔE is comparable to kT (Fig. 3c). Let us take an example. In the simple bipyridyl complex Pt(bpy)Cl₂ 3, the d–d state is the lowest-lying excited state, and the complex does not emit at room temperature [12]. In the analogous complex of the ester-substituted bipyridine, Pt{3,3'-(MeCO₂)₂-bpy}Cl₂ 4, the lowest-lying excited state is now the MLCT state, but the d–d state lies only a little higher in energy, offering the former a non-radiative decay pathway and also rendering this complex non-emissive at room temperature [13].

Fig. 3

(a) Simple ligand field splitting diagram for metal d orbitals in a square planar d ⁸ complex. The z axis is, by convention, perpendicular to the plane of the complex and the M–L bonds lie along the x and y axes. Although the ordering of the lower energy levels depends on the ligand set (e.g. relative importance of σ- and π-effects), the d _{x 2–y2} is always the highest. (b) The displaced potential energy surface of the d–d excited state formed by population of the d _{x 2–y2} orbital. (c) The d–d state can provide a thermally activated decay pathway for other, lower-lying excited states if ΔE is comparable to kT. Thick arrows represent absorption of light; thin ones indicate vibrational relaxation and non-radiative decay

3.1 Tuning Excited States to Optimise Luminescence Efficiencies

The above discussion leads naturally to the conclusion that the most emissive platinum(II) complexes will be those in which (1) the lowest-energy excited state is not a metal-centred d–d state but rather an LC or CT state, and (2) where the energy gap between the lowest excited state and the d–d state is sufficiently large as to prevent thermally activated decay via the latter.

For example, platinum(II) porphyrins are perhaps the oldest known class of luminescent Pt(II) complexes. They emit deep into the red region (e.g. λ _max = 641 nm, Φ _lum = 0.6 for 1 [14]). Clearly the emissive, porphyrin-based π–π* state is sufficiently low in energy that activation to the d–d state is ruled out at room temperature. However, relying on the low-energy of the emissive state to disfavour non-radiative decay is clearly not an option if higher-energy blue- or green-emitting complexes are sought! In such cases, it is necessary to work on ensuring that the d–d states in the complex are kept at high energies [11]. In practice, this means that strong-field ligands need to be present in the coordination sphere of the metal ion.

For example, in contrast to Pt(bpy)Cl₂ mentioned above, complexes of the form Pt(bpy)(–C≡C–Ar)₂ are normally quite strongly emissive in solution at room temperature [15]. The difference can be attributed – at least in part – to the stronger ligand-field associated with the acetylide compared to the chloride co-ligands. Similarly, cyclometallated ligands, particularly those offering the metal ion a synergistic combination of a π-accepting heterocycle (e.g. pyridine) with the strongly σ-donating cyclometallated carbon, lead to a strong ligand field. This accounts, for example, for [Pt(N^C-ppy)(N^N-bpy)]⁺ being luminescent at room temperature, in contrast to [Pt(N^N-bpy)₂]²⁺, which is not (ppyH = 2-phenylpyridine) [16]. A more comprehensive discussion of these principles is provided in a recent contribution to the sister volume of this series [11].

In this chapter, we shall overview some of the main classes of platinum(II) complexes that are luminescent under ambient conditions, highlighting those which have been shown to be potentially useful OLED dopants [17]. The emphasis is on examples from the past 3 years, although naturally not to the exclusion of important and informative earlier original examples.

4 PtL₂X₂ Complexes (L ═ Neutral 2e⁻ Donor Ligand, X ═ Anionic Ligand)

Many of the complexes which fall into this class are Pt(N^N)-based compounds, where N^N is a bidentate diimine ligand such as bipyridine or phenanthroline and derivatives thereof. As noted in Sect. 3, those where X = Cl are rarely significantly emissive, because the d-d state is either the lowest-energy excited state or is thermally accessible from a lower-lying CT or LC state [13].

4.1 X ═ –C≡N

The substitution of the halides by strong-field cyanides (X = –C≡N) lowers the energy of the highest-occupied metal-centred orbitals and raises the vacant d _{x 2–y2} orbital, such that Pt(N^N)(CN)₂ complexes normally emit from π–π* states. Many of these complexes also form emissive excimers [18–20]. For example, Pt(dpphen)(CN)₂ 5 displays green emission centred at 520 nm in dilute CH₂Cl₂ (10⁻⁵ M), but increasing the concentration leads to the appearance of a red emission band, centred at 615 nm, which grows in at the expense of the green band with an identical excitation spectrum [19]. The red emission is attributed to an excimer analogous to well-known examples amongst planar organic compounds such as pyrene, and formed from the bimolecular interaction of an excited monomer with a ground-state monomer [21].

Very recently, Kato and colleagues prepared Pt(dC _n bpy)(CN)₂ 6 complexes bearing long alkyl chains, where dC _n bpy represents bipyridine substituted at the 4 and 4′ positions by –C₉H₁₉ (n = 9) or –C₁₁H₂₃ (n = 11) [22]. These compounds could be prepared by treatment of the respective ligands with platinum dicyanide in a mixture of DMF and aqueous ammonia. The long chains render the complexes soluble in a wider range of solvents than the parent systems, and to higher concentrations, which has allowed the solvent dependence of excimer formation to be investigated. In polar solvents such as methanol and acetonitrile, exclusively monomer emission is observed over the concentration range 10⁻⁵ to 10⁻³ M. Over the same range in chloroform, the emission switches from monomer to excimer, whilst in toluene solution, the excimer is observed even at 10⁻⁵ M. Evidently, the reorganisation of the molecules to form the excimer is favoured by solvents of low polarity. The difference is strikingly revealed by the change in the emission spectrum of a 5 × 10⁻⁵ M solution of the n = 9 complex in different ratios of CHCl₃/toluene (Fig. 4).

Fig. 4

Change in the emission spectrum of [Pt(dC₉bpy)(CN)₂] (6, n = 9) (5 × 10^–5 M; λ _ex = 320 nm) in different ratios of CHCl₃/toluene, from 100% CHCl₃ to 100% toluene in 10% steps [22]

In an unusual recent development, Kunkely and Vogler have investigated the excited state properties of Pt(COT)(CN)₂ 7 (COT = cyclo-octatetraene) [23]. The COT ligand can be thought of as an analogue of a diimine, in that it has relatively low-energy π* orbitals. Indeed, the complex in the solid state displays a long-wavelength absorption maximum centred at 430 nm and intense orange emission at 572 nm, attributed to the singlet and triplet MLCT {Pt^II → π*(C₈H₈)} states respectively (Fig. 5). Unfortunately, in solution, the complex is of limited stability and degrades with release of COT.

Fig. 5

Excitation (λ _em = 572 nm) and emission (λ _ex = 400 nm) spectra of solid Pt(COT)(CN)₂ 7 at room temperature. Reprinted from [23] with permission from Elsevier

In addition to excimer formation, square planar platinum(II) complexes are frequently able to participate in ground-state intermolecular interactions through their axial positions. In some cases, this leads to oligomerisation through overlap of the d _z2 orbitals that are orthogonal to the plane of the complex [24, 25]. The combination of isonitriles and cyanides as ligands for Pt(II) has been extensively studied by Mann and co-workers in their work on vapochromic sensors [26, 27]. The intermolecular interactions in double salts of the type [Pt(CNR)₄][Pt(CN)₄] and the related neutral complexes [Pt(CNR)₂(CN)₂]₂ are influenced by the presence of volatile organic solvents, leading to characteristic changes in colour [28]. Recently, Wang and Che have described luminescent micro- and nano-wires that form by self-assembled aggregation of [Pt(CN^tBu)₂(CN)₂] directed by Pt···Pt interactions [29]. This complex itself is non-emissive in acetonitrile solution at room temperature, but displays bright green luminescence from the rod-like crystalline form. The corresponding X-ray structure reveals infinite linear stacks comprised of pairs of complexes, with a torsion angle of 125.20° between them about the stack axis, and close Pt···Pt distances of 3.354(1) Å. Evaporation of an acetonitrile solution in air on a silicon substrate led to luminescent wires of diameter approximately 25 μm and lengths as great as 1,000 μm or more, which could be readily visualised by emission microscopy (Fig. 6a). Wires of smaller diameter could be prepared by prior dilution of the acetonitrile solution with water; an example visualised by scanning electron microscopy is shown in Fig. 6b. The luminescence emitted by the wires (λ _max = 562 nm at 298 K) resolves out into two components at 77 K (λ _max = 545 and 597 nm). The former is attributed to the dimeric Pt₂ ³[5dσ* → 6pσ] excited state and the latter to trimeric and tetrameric species, an assignment supported by recent density functional theory (DFT) calculations [30].

Fig. 6

(a) Emission microscopy image of luminescent [Pt(CN^tBu)₂(CN)₂] wires formed on a silicon substrate by slowly evaporating a 10⁻⁴ M solution in MeCN. (b) Scanning electron micrograph of thinner wires of the same compound formed by injecting 50 mL of a 9.6 × 10⁻³ M solution in MeCN into 10 mL of water, followed by transfer onto a silicon substrate. Note the different magnification in the two images. Reprinted from [29] with permission from Wiley

4.2 X ═ Organometallic Carbon

Organometallic derivatives of the form Pt(N^N)(–Ar)₂, incorporating Pt–aryl bonds, have also been studied. They are accessible by reaction of the diimine ligands (e.g. bpy or phen) with Pt(COD)(aryl)₂ precursors, in turn formed from Pt(COD)Cl₂ and the corresponding aryl-lithium reagent [31]. The bis-mesityl complex 8 emits at 660 nm in toluene solution at room temperature from a ³MLCT state [32]. Clearly, the strongly σ-donating aryl groups serve to lower the energy of the excited state. The mesityl complexes are unusual amongst square planar platinum(II) compounds in that they can be reversibly oxidised to remarkably persistent d ⁷ Pt(III) species, an effect attributed to the effective protection of the axial positions by the two mesityl groups [31].

More recently, related compounds containing perfluorinated aryl rings have been prepared, e.g. 9 [33]. Like the mesityl systems, the aryl rings are twisted by about 70° with respect to the PtN₂C₂ square plane in the solid state structures (Fig. 7). The emission of Pt(dpphen)(Ar^F)₂ 9a (λ _max = 515 nm in CH₂Cl₂) is substantially blue-shifted compared to the mesityl compound 8, which can be readily rationalised in terms of the electron-withdrawing influence of the fluorine substituents in lowering the energy of the metal-centred HOMO. No emission could be detected from the analogous complex incorporating methyl substituents at the 2 and 9 positions of dpphen, 9b. The X-ray structure of this complex reveals a bent structure (Fig. 7) and slightly longer Pt–N bonds owing to steric hindrance of the methyl groups, both of which features will lead to a weaker ligand field at the metal and to a less rigid structure, promoting non-radiative decay. The fluorine substituents aid sublimation, and a multi-layer OLED incorporating 6% by mass of 9a within a CBP host emissive layer has been prepared by thermal deposition; it displays an external quantum efficiency of 2.1% at 100 cd m⁻².

Fig. 7

Molecular structures of 9a and 9b in the solid state from X-ray diffraction analysis [33]

Fekl and co-workers have recently reported the first examples of diimine-coordinated platinum complexes with 9,10-dihydroplatinaanthracene co-ligands [34]. These compounds are metallacycles: the Pt ion replaces one carbon atom within the carbocyclic system (anthracene). Reaction of the new organometallic oligomeric platinum precursor {[H₂C–(C₆H₄)₂]Pt(SEt₂)} _n (n = 2,3) with diimines such as bipyridines, phenanthrolines and biquinoline provides easy access to a whole range of new complexes incorporating the metallacycle. The lowest-energy absorption bands of the bipyridyl series 10a–e increase in energy as the diimine becomes more electron-rich, in line with charge-transfer-to-diimine character to the lowest spin-allowed excited state (Fig. 8). This is supported by electrochemistry and DFT calculations, which suggest that the HOMO is based primarily on the platinacycle. Although none of the compounds prepared to date display detectable emission in solution at room temperature, they emit brightly in the solid state, showing a trend in emission energies that is largely consistent with that in absorption.

Fig. 8

Normalised charge-transfer-to-diimine absorption bands for metallacycles 10a–e in MeCN at room temperature. Identity of R: a = Cl, b = H, c = t-Bu, d = MeO, e = NMe₂. Reprinted from [34] with permission from ACS

4.3 L₂X₂ ═ Salen and Derivatives

Oxygen ligands on their own are generally not suitable for forming stable complexes with the soft Pt(II) ion, but in combination with other ligands such as diimines, complexes with high thermodynamic stability can be obtained. Shagisultanova reported that the well-known tetradentate ligand, N, N'-bis(salicylidene)-1,2-ethylenediamine or salen, forms a highly luminescent complex with Pt(II), 11 [35]. The emissive excited state is probably one of d(Pt)/π(O) → π*(Schiff base) character. Che and co-workers investigated this complex and its tetramethylethylenediamine analogue as sublimable, green-emitting OLED dopants [36]. At the optimised dopant concentration of 4 wt%, a maximum external quantum efficiency of 11% was achieved at 8.5 mA cm⁻². The same team has also explored complexes of related ligands based on dpphen or tbbpy appended with ortho-phenoxy groups, which prove to be superior in terms of solution quantum yields; e.g. Φ = 0.6 for 12 in DCM compared to 0.19 for 11 in MeCN [37].

Wong and co-workers have recently described the result of intriguing intermolecular packing effects in a dimethoxy-substituted salen complex 13 [38, 39]. For example, different solvates have been crystallised, 13.H₂O and 13.DMF, which are orange and red respectively. The very different packing arrangements in the two crystals are shown in Fig. 9a, b. Meanwhile, the self-assembly of 13 with K₂Pt(CN)₄ or K₂Pd(CN)₄ leads to compounds [K₂(13)₂M(CN)₄] (M = Pd or Pt, isostructural; Fig. 9c). K–O coordination in the crystals of these compounds brings the 13 units closer together than in the two solvates, probably accounting for the greater red component in the solid-state emission spectrum of the former compounds [38].

Fig. 9

Packing of the molecules in the crystals of (a) 13.H₂O and (b) 13.DMF, from X-ray diffraction analysis. Hydrogen atoms are omitted for clarity. (c) The packing and one-dimensional helical chains observed in the structures of [K₂(13)₂ M(CN)₄] (M = Pd or Pt). Reprinted from [38] with permission from Elsevier

5 Pt(N^N^–)-Based Complexes: ‘Pseudo-Cyclometallates’

Whereas pyridines and quinolines necessarily bind to metal ions as neutral two-electron ‘L’ donors, heterocycles such as pyrazole, indazole, imidazole and triazole can do so either via a neutral nitrogen atom or through deprotonation of a ‘pyrrolic’ nitrogen, as does pyrrole itself. Bidentate ligands that incorporate each type of ring, for example 3-(2-pyridyl)pyrazole, can thus frequently bind either as N^N or N^N^– ligands (Fig. 10). In the latter case, the coordination offered to the metal is essentially analogous to that of a cyclometallating ligand such as 2-phenylpyridine (ppy), and the resulting complexes could be construed as pseudo-cyclometallates, in which the N^– atom resembles a metallated carbon. Since such ligands should exert a strong ligand field, a beneficial effect on luminescence efficiencies might be anticipated.

Fig. 10

The tautomeric forms of 3-(2-pyridyl)-pyrazole can allow it to coordinate to a metal M as either a neutral N^N-binding ligand, or as an anionic N^N^– ligand, analogous to a cyclometallating N^C ligand

Chi and co-workers have explored the chemistry of platinum(II) with a series of 3-(2-pyridyl)pyrazole, 3-(pyrazyl)pyrazole and 3-(2-pyridyl)triazole ligands, and have noted that their luminescence quantum yields in solution vary widely according to the identity of substituents on the rings [40]. For example, the tert-butyl-substituted complexes 14 and 15 are amongst the most intensely luminescent platinum(II) complexes known (Φ = 0.82 and 0.86; τ = 2.4 and 2.1 μs respectively). In contrast, the analogues 16a and 16b, incorporating fluorinated substituents, and the triazole systems 17a and 17b display only very weak and short-lived emission under the same conditions (Φ < 10⁻³; τ < 10 ns), possibly due to severe self-quenching. Nevertheless, bright emission is observed from thin solid films of these compounds, probably due to aggregation to give a Pt₂ dσ* → π*(N^N) excited state. This assignment is supported by the short Pt···Pt contacts (3.442 Å) in the crystal structure of 16b, compared to a displaced arrangement of the molecules in the structure of 15, where Pt···Pt distances are too long for significant interaction of d _z2 orbitals. Both types of complexes have been successfully incorporated into electroluminescent devices by doping into a CBP host; e.g. an external quantum efficiency of 6% was observed in such a device employing 16a.

The change from N^N to N^N^– and the beneficial effect this can have on luminescence efficiency is dramatically illustrated by the platinum(II) complexes of a pair of isoquinolyl–indazole ligands also investigated by Chi [41]. The complexes in which these ligands are bound as neutral N^N ligands, 18 and 19, are non-emissive in solution, just like Pt(bpy)Cl₂. However, upon reaction with anionic ligands such as picolinate or a pyridylpyrazolate, deprotonation of the indazolic N–H occurs to give N^N^–-bound complexes, e.g. 20 and 21. These complexes are luminescent in solution at room temperature, a change that can be attributed to the increase in ligand field strength that accompanies deprotonation. The 1-linked isomers are consistently better emitters than the 3-linked isomers (e.g. Φ _lum = 0.64 and 0.035, τ = 8.2 and 0.85 μs for 20 and 21 respectively, in CH₂Cl₂ at room temperature), which seems to be related primarily to faster non-radiative decay in the latter.

Pyrroles can function in a similar way to the deprotonated pyrazoles of the above systems, when incorporated into chelating ligands reminiscent of salen, as demonstrated by Che and co-workers [42]. They prepared the complexes 22–24 which display the emission spectra shown in Fig. 11. Solution quantum yields of 22 and 23 are of the order of 0.10 (in MeCN at 298 K), while the shift to lower energy in the phenylene-based system 24 – associated with the more extended π-conjugated system – is accompanied by reduction in Φ _lum to 0.001. OLEDs incorporating 22 and 23 doped into a CBP emissive layer have been fabricated. In the case of 22, the spectral output is dependent on dopant concentration, λ _max increasing from 580 to 620 nm on going from 0.8 to 6.0 wt%. This change is attributed to excimer or oligomer formation at the higher loadings. Notably, the colour coordinates (0.62, 0.38) of the 6% system are close to those for pure red light (0.65, 0.35). No such concentration dependence is observed for 23, probably because the methyl groups inhibit the close approach of the molecules that is necessary for the oligomer or excimer to form.

Fig. 11

Structural formulae of 22–24 and their absorption and photoluminescence spectra in MeCN at 298 K. Reprinted from [42] with permission from RSC

Platinum-containing heteropolynuclear complexes based on deprotonated pyrazoles have recently been reported by Umakoshi et al. [43]. These intriguing compounds have formulae [Pt₂M₄(μ-Me₂pz)₈] (M = Ag or Cu; 26 and 27 respectively), and are prepared from the corresponding protonated systems (M = H, 25) (Fig. 12) {Me₂pz = 3,5-dimethylpyrazolate). The compounds 25, 26 and 27 display yellow, blue and orange luminescence (Φ _lum = 0.06, 0.85 and 0.28) respectively, in the solid state at 295 K. They also emit in solution, although substantially more weakly, probably because of significant structural changes accompanying the formation of the excited state, as suggested by the large red-shifts in solution compared to the solid state (Fig. 13). DFT calculations highlight the importance of metal-metal bonding in the frontier orbitals and indicate that the microsecond emission of 26 and 27 can be assigned to states of ³[Pt₂ → Pt₂Ag₄] and ³[Cu(d) → Pt₂Cu₄] character respectively.

Fig. 12

The synthesis of 26 and 27 by treatment of 25 with Ag⁺ or Cu⁺ respectively

Fig. 13

Normalised emission spectra of 25, 26 and 27 in the solid state (no prime) and in CH₂Cl₂ at 295 K (prime), λ _ex = 266 nm. Reprinted from [43] with permission from ACS

6 Pt(N^C)-Based Complexes (N^C = Bidentate Cyclometallating Ligand)

6.1 The 2-Arylpyridines and Analogues: Versatile Cyclometallating Ligands for Colour Tuning of Platinum(II) Complexes

The bis-cyclometallated complex Pt(N^C-thpy)₂ 28 (thpyH = 2-thien-2-ylpyridine), reported by von Zelewsky in the mid-1980s [44], was one of the first clear-cut examples of a simple platinum(II) complex that phosphoresces in fluid solution at room temperature (λ _max = 578 nm, τ = 2.2 μs in a PrCN/MeCN mixture) [45]. Barigelletti et al. showed, by means of a study of the temperature dependence of emission, that the influence of cyclometallation is to displace the d–d state to about 3,700 cm⁻¹ above the emissive state, ensuring that thermally-activated non-radiative decay is blocked off [46]. Although not thermally stable and hence not suitable for vacuum deposition methods of OLED fabrication, this complex and the trimethylsilyl derivative 29 has been incorporated into devices by spin-casting with TPD as a host material, and an EL efficiency of 11.5% has been achieved with 29 [47, 48].

The formation of such bis-cyclometallated complexes incorporating two such N^C ligands is not trivial, requiring the use of lithiated ligands, the formation of which is intolerant of many functional groups. In fact, complexes with just one such ligand and a second, coordinating (as opposed to metallated) bidentate ligand are excellent alternatives and are normally more readily prepared. For example, it is often possible to treat the N^C ligand with one equivalent of K₂PtCl₄ to generate a dichloro bridged dimer, (N^C)Pt(μ-Cl)₂Pt(N^C), which can then be cleaved with a variety of monodentate or bidentate ligands [49]. The dimers are usually formed readily in mixed solvent systems such as ethoxyethanol/water at 80°C. Reaction times are usually of the order of 24 h, although Ghedini and co-workers have recently reported that microwave irradiation can greatly speed up the reaction, provided that the temperature does not rise beyond about 65°C, which seems to induce degradation of the platinum salt [50].

Treatment with bidentate diamines, diimines or bis(pyrazolyl)borates gives [Pt(N^C)(N^N)]⁺ monometallic complexes [16, 51]; bidentate anionic ligands L^X such as β-diketonates give neutral Pt(N^C)(O^O) [52]. Unidentate ligands can also be introduced to give, e.g. [Pt(N^C)Cl₂]^–, Pt(N^C)(CO)Cl, Pt(N^C)(CO)SR [16, 53, 54]. In fact, the choice of the other ligands is crucial to luminescence efficiency. For example, the ligand field strength in [Pt(N^C)Cl₂]^– is inadequate to inhibit non-radiative decay via the d–d state, whereas diimine complexes such as [Pt(N^C)(bpy)]⁺ are significantly emissive at room temperature. In the latter case, the bpy also influences the nature of the excited state, since the LUMO is largely localised on this ligand, leading to a d(Pt) → π*(N^N) excited state [16].

The β-diketonate class Pt(N^C)(O^O) (e.g. O^O = acac) have proved to be very popular, as the O^O ligand is easily introduced and normally does not influence the excited state energy significantly, so that the properties are essentially those associated with the N^C ligand. Thompson and co-workers carried out a landmark study of such systems with acac or its tert-butyl analogue dipivaloylmethane (dpm), in which 23 different N^C-coordinating ligands were investigated [52]. This study revealed that the d(Pt)/π(N^C) → π*(N^C) emission of these systems can be tuned over a wide range by relatively simple structural modification of the N^C ligand, in a way that is quite similar to that achieved over the past decade with N^C-based iridium(III) complexes. Briefly, electron-withdrawing substituents in the aryl ring and electron-donating substituents in the pyridine ring lower the HOMO and raise the LUMO respectively, leading to a shift to the blue. In contrast, extending the conjugation in the heterocycle through the use of quinolines, or using benzothiazole in place of phenyl as the cyclometallating ring, shifts the emission to the red. The extremes are represented by 30 and 31 which, at 77 K, have emission maxima of 440 and 600 nm respectively. Although many of the complexes also emit quite strongly at room temperature, this is not true for all; e.g. for 30, Φ _lum < 10⁻³, possibly due to a small energy gap between the emissive and higher-lying d-d state.

A series of related complexes incorporating styryl pendants 32a–d (Fig. 14) have been investigated by Guerchais and co-workers, which display some rather intriguing properties arising from the possibility of trans–cis isomerisation of the C═C bond [55]. Emission is weak at room temperature, due to competitive C═C photoisomerisation, as confirmed by ¹H NMR spectroscopy. Little effect of the pendant on the emission energy is observed under these conditions, suggesting that the emissive state at room temperature is decoupled from the styryl group, perhaps indicative of a half-twisted –CH═CH– unit.

Fig. 14 The structures of the styryl-appended complexes 32a–d and the long wavelength region of their emission spectra, where the E isomers emit, in an EPA glass at 77 K; λ _ex = 400 nm

In contrast, in a rigid frozen glass at 77 K, two sets of bands with similar, well-defined vibrational progressions are observed, one in the region 460–560 nm, and the second set at substantially lower energy in the range 570–800 nm. These are assigned to the cis (Z) and trans (E) isomers respectively, evidence for which includes the change in their relative intensities according to the excitation wavelength. The former decrease in relative intensity as the excitation wavelength is increased from 350 to 430 nm, which correlates with the longer-wavelength absorption by the E isomer. The energy of the E bands is influenced by the substituent, as shown in Fig. 14, which reveals that the emission can be pushed right out to the far red by using an amine pendant [55].

In some complexes, the energy of the L^X ligand in Pt(N^C)(L^X) may be lower than that associated with the Pt(N^C) unit, in which case emission is likely to emanate from an L^X-based excited state. An example is provided by 8-quinolinol (the anion of 8-hydroxyquinoline, HQ) as a ligand, e.g. complex 33. Here weak emission deep into the red is observed in solution at room temperature, which can be further shifted to the NIR upon introducing substituents into the Q ligand [56, 57]. The emitting state has ILCT character, with a HOMO localised on the phenolate and LUMO on the pyridine ring of the same ligand, just as for the homoleptic complex Pt(quinolinol)₂ complexes investigated 30 years ago [58]. Despite the low quantum yield in solution (<0.01) and in powdered form (0.003), complex 33 has been doped into the emissive CBP layer of a multilayer OLED, giving a device displaying saturated red output (CIE coordinates 0.71, 0.28), albeit of efficiency that is too low to be practicable [59].

6.2 Excimers and Introduction to WOLEDs

As noted in Sect. 4.1, sterically unencumbered platinum(II) complexes may sometimes form excimers and/or aggregates, which emit at lower energy than the isolated monomeric molecules. In terms of applications to OLEDs, the combination of monomer and excimer emission from a single metal complex as dopant may offer an intriguing way forward in the development of white light-emitting devices (WOLEDs) [60]. Provisionally these have vast potential as efficient replacements of incandescent light bulbs and fluorescent strip lighting: around 22% of energy consumption in the U.S. is estimated to go on lighting [61]. White light is normally generated by using three separate emitters – red, green and blue [62]. If all three are combined within a single emitting layer, it can be difficult to control energy transfer from the higher energy blue to the green to red dye: energy tends to be ‘short-circuited’ to the lowest energy emitter. Segregation of the emitters into different layers can circumvent this problem, but this requires a more elaborate device architecture [63]. Thompson and Forrest put forward the novel approach of making use of a single phosphorescent dopant emitting simultaneously from monomer (blue–green) and excimer (green–red) states [60]. Clearly, Pt(II) complexes are potentially excellent candidates for this.

The trick is to achieve the right balance of excimer and monomer at the right sort of doping level, and this can be controlled to some extent through sterics. For example, photoluminescence spectra of a CBP host doped with increasing concentrations of Pt(N^C-F₂ppy)(acac) demonstrate that, at a low doping level of 1.5%, undesired residual emission from the host is observed in addition to that of the monomeric Pt complexes [64]. As the doping level is increased, the excimer emission rapidly appears and dominates at levels >8%, which is too low to achieve colour-balanced emission in a practicable device. If the complex is made more bulky to inhibit partially excimer formation, the onset of excimer emission can be limited to higher concentrations. The dpm (dpmH = dipivaloylmethane) derivative is too bulky: almost exclusively monomeric emission is observed even at 20% doping level. The mhpt complex, on the other hand, offers just the right level of steric bulk for balanced emission at 10% doping, and this behaviour has been successfully extrapolated to an electroluminescent device (Fig. 15) (mhptH = 6-methyl-2,4-heptanedione). The CIE coordinates of this device (0.36, 0.44) are close to those for white light (0.33, 0.33), with a maximal efficiency of 3.3 ± 3 photons/electron at 0.5 cd m⁻² [64].

Fig. 15

Electroluminescence spectra and CIE coordinates for Pt(F2ppy)(mhpt) incorporated into the emissive layer of the multilayer device represented schematically in the inset, at the % dopings by mass indicated. Reproduced from [64] with permission from RSC

6.3 Multifunctional Complexes

In most devices, the emissive layer is sandwiched between hole- and electron-transporting layers, in order to achieve balanced injection and transport of holes and electrons to optimise charge recombination within the EL layer. Wong et al. have been investigating multifunctional Pt emitters in which oxadiazole and triarylamine units are covalently linked to a Pt(ppy)(acac) emissive centre, to act as electron- and hole-transporting units respectively, e.g. 34 [65]. These sublimable compounds have been used to prepare neat-emissive-layer devices by vacuum evaporation between an ITO/CuPC anode and Ca/Al cathode. Molecules such as 34 display fluorescence from the arylamine as well as Pt-based phosphorescence, both in solution and when doped in the CBP emissive host. The ratio of fluorescence to phosphorescence can be controlled according to either the applied voltage (the blue colour intensity increases relative to the orange as the voltage increases), or the concentration of the dopant (intermolecular quenching of triplet excitons at high loadings leads to an increase in the proportion of blue component). Under appropriate conditions, the combination of the two contributions can be tuned to give WOLEDs, offering an alternative to the monomer-excimer approach [66].

Structurally analogous molecules incorporating fluorene instead of the oxadiazole have been prepared by the same group 35a–c [67]. Again, the introduction of diarylamino substituents onto the fluorene leads to ILCT singlet fluorescence in addition to the Pt-based phosphorescence (Fig. 16). Notably, the 77 K phosphorescence lifetimes of the substituted systems are significantly longer than the unsubstituted analogue, in line with a smaller contribution of metal character in the excited state of these more extended systems, even though the triplet excited state energies are essentially unaffected.

Fig. 16

Absorption and photoluminescence spectra of 35a–c in CH₂Cl₂. Reprinted from [67] with permission from ACS

Finally, we note that Marder and co-workers have recently reported on the covalent incorporation of Pt(N^C)(O^O) complexes into a polymer by ring opening metathesis copolymerisation of norbornene-appended complexes, e.g. 36, with a norbornene-appended carbazole 37 [68]. Such an approach could provide a solution to some of the problems associated with phase separation in blends of small-molecule phosphors and polymers. The photoluminescence spectra of the films formed in this way display both monomeric and excimer/aggregate emission (Fig. 17), as also proves to be the case in the electroluminescent devices.

Fig. 17

Photoluminescence spectrum of a film of copolymer 37-co-36, and corresponding spectra of monomer 36 as a thin film and in dilute solution in THF. Reprinted from [68] with permission from ACS

7 Complexes with Terdentate Ligands

The d–d excited states of d ⁸ complexes comprising bidentate ligands are unstable with respect to distortion away from the D _4h symmetry of the ground state towards a D _2d symmetry, in which the plane of the bidentate ligand is twisted relative to that containing the other two ligands [69]. As highlighted in Fig. 3, excited state distortion is associated with increased non-radiative decay, with an adverse effect on luminescence quantum yields. Terdentate ligands analogous to bpy and ppy, able to bind via three rings instead of two in a planar conformation, should in principle inhibit such distortion owing to a more rigid binding [70]. However, the bite angle associated with many such ligands, e.g. terpyridine (tpy), is not ideal for larger metal ions, leading to a weaker ligand field compared to that which might be anticipated. Thus, [Pt(tpy)Cl]⁺, for example, is not emissive at room temperature, because the d–d state is low in energy [71]. Again, however, the introduction of stronger-field co-ligands in place of Cl can circumvent this problem, and a wide range of emissive complexes based on the [Pt(tpy)(–C≡C–R)]⁺ structure have been investigated. The reader is directed to recent reviews on such systems by Castellano et al. and by Wong and Yam [15, 72]. In this contribution, our focus is on charge-neutral systems, which are more relevant to the topic of OLEDs. A variety of charge-neutral Pt(II) complexes have been obtained by employing cyclometallating terdentate ligands in combination with an anionic fourth ligand, selected examples of which are discussed in the sections that follow.

7.1 Platinum(II) Complexes with N^N^C-Binding Ligands

Che and co-workers have pioneered much of the work on cyclometallated Pt(II) complexes with the N^N^C-coordinating ligand, 6-phenylbipyridine (phbpyH) and derivatives [73]. Of particular interest is their comprehensive study of around 30 such complexes incorporating σ-alkynyl ligands in the fourth coordination site of the metal ion, [Pt(N^N^C)(–C≡C–R)] [74]. The majority of the complexes are emissive in solution at room temperature, giving featureless spectra ranging from green to red. The emission in most of the systems is attributed to a ³MLCT (d_Pt → π*_NNC) state, an assignment that is supported by the influence of structural modification of both the N^N^C ligand and the acetylide on the emission properties. For example, for a series of p-substituted aryl acetylides 38, the emission maximum shifts to higher energy as the substituent X becomes more electron-withdrawing, and the metal-based HOMO is lowered in energy (Fig. 18). A good negative linear correlation of log Σk _nr with the emission energy illustrates that the energy gap law is obeyed within this series (Fig. 18, inset). Meanwhile, introduction of electron-donating tert-butyl groups into the 4-positions of the pyridyl rings of the N^N^C ligand shifts the emission energy to the blue (39, R = t-Bu vs. R = H), whilst an ester at this position (39, R = –CO₂Et) has the opposite effect, reflecting the contrasting influence of these substituents in raising and lowering the LUMO energy respectively. Substituents in the cyclometallating phenyl ring were found to have rather little influence, but changing from phenyl to thienyl or furanyl rings (40, X = S or O), led to a substantial red shift in emission, which can be attributed to the more electron-rich nature of these heterocycles. The complexes are thermally stable and a number of them have been incorporated successfully into OLEDs.

Fig. 18

Normalised emission spectra of complexes of type 38, carrying the different X groups indicated, in dichloromethane at 298 K. In order of increasing λ _max: X = NO₂ < Cl < H = F < Me < MeO. Inset: plot of ln Σk _nr vs the energy corresponding to the respective emission maxima. Reprinted from [74] with permission from ACS

Interestingly, in some cases, a switch to ³LC excited states localised on the acetylide or on the aryl group of the acetylide can be achieved. For example, Fig. 19 shows the influence of polyacetylide ligands: on going from the n = 3 to n = 4 system 41, a dramatic switch to a highly structured acetylide-localised state is observed. Similarly, when a 2-pyrenylacetylide ligand is used, the lowest-lying excited state is the ³LC state associated with the pyrenyl moiety, which emits deep into the red (λ _max 664 nm).

Fig. 19

The influence of extending the polyacetylide ligand in the generic structure shown, revealing the switch from ³MLCT to ³LC emission that occurs at n = 4. Reprinted from [74] with permission from ACS

A somewhat related complex incorporating a pyrenyl unit has been reported recently by Li and Yang, but in this case the pyrenyl unit is substituted onto the central 4-position of the N^N^C ligand, 42 [75]. In this compound, dual emission is observed from both ³MLCT (d _Pt → π*_NNC) and pyrenyl-based ³π–π* states, both at 77 K and – unusually – at room temperature. The grow-in and decay kinetics of the two bands, in conjunction with transient absorption spectroscopy, reveal that the pyrenyl ³π–π* state is largely populated directly by intersystem crossing from the ¹MLCT state, as is the ³MLCT state, whereas the triplet–triplet internal conversion of the ³MLCT state to the pyrenyl ³π–π* state is of negligible importance.

Fillaut et al. have also observed emission from a ‘remote’ π–π* state in the complex 43, where long-lived emission (τ ∼18 μs) centred around 570 nm is associated with the flavonol unit, whose triplet emission is promoted by coupling to the platinum centre [76]. Interestingly, when the flavonol binds divalent metal ions, particularly Pb²⁺, the relative energies of the excited states change such that the two parts of the molecule become decoupled, and emission then switches to being flavonol-based fluorescence (Fig. 20).

Fig. 20

Luminescence spectral changes of 43 (10⁻⁵ M) in MeCN upon addition of Pb(ClO₄)₂ (λ _ex = 393 nm) [76]

McMillin and co-workers have recently reported on the platinum(II) complex of an N^N^C-coordinating ligand that incorporates a phenazine in the backbone, 44, together with its cationic N^N^N analogue 45 [77]. They investigated the influence of Lewis acids and bases on the emission. The emission of both complexes is quenched by Lewis bases, as frequently observed for simple Pt(II) complexes, and rationalised on the grounds of sterically accessible attack at the coordinatively unsaturated Pt centre. However, the emission of 44, but not of 45, is also quenched by acid. This difference is attributed to the greater degree of charge-transfer character in the cyclometallated system, which is also manifest in its substantially shorter excited state lifetime (τ = 270 ns in chloronaphthalene compared to 5 μs for 45 in dichloromethane).

7.2 Platinum(II) Complexes with N^N^O-Binding Ligands

As noted in Sect. 4.3, the combination of diamine or diimine and phenolic donors in salen and related ligands can lead to emissive complexes. In the same way, the aryl ring of N^N^C-based complexes can be replaced by an ortho-phenolate. Thus, a range of luminescent N^N^O-coordinated complexes based on the 6-(2-hydroxyphenyl)-2,2'-bipyridine, have been synthesised by Che and co-workers, 46 (where Y = H, Me or t-Bu; Z = H or F) [78]. In fact, the parent ligand was initially isolated as a side product of the synthesis of the bpy-based analogue of the tetradentate ligand in complex 12, but the terdentate ligand also proved successful at forming thermally stable, emissive complexes. Broad orange–red emission at room temperature again originates from a mixed d_Pt/p_O → π*_N^N excited state. When incorporated into OLEDs, good performance yellow devices were obtained. Although dopant concentration did not affect the device colour, the presence of fluorine and tert-butyl at positions Z and Y respectively was found to give the highest efficiency.

7.3 Platinum(II) Complexes of N^C^N-Binding Ligands

Our group has been investigating complexes of 1,3-di(2-pyridyl)benzene (dpybH) and derivatives. The parent system 47 (Y = Z = H) is the isomer of the N^N^C-coordinated complex (Sect. 7.1). However, the positioning of the cyclometallating aryl ring in the central rather than lateral position within the terdentate system leads to improved luminescence efficiency (e.g. φ = 0.60 for the parent compound), probably due to the increased ligand field strength associated with the shorter Pt–C bond [79]. The highly structured emission that is scarcely shifted in position nor elongated in lifetime upon cooling to 77 K is suggestive of a primarily ligand-centred π–π* state, in which the contribution of the metal is nevertheless sufficient to promote the triplet emission. These conclusions are supported by DFT calculations [10], which reveal a HOMO that spans the cyclometallating ring, the metal and the chloride co-ligand, and a LUMO that is localised on the pyridine rings.

This localisation of frontier orbitals also accounts for the influence of substituents on emission energy. For example, the introduction of aryl substituents into the central 4-position (Y) of the central ring leads to a lowering in the excited state energy, both the singlet state (E _abs) and triplet (E _em). A good linear correlation of both E _abs and E _em is observed with the oxidation potential, which can be understood in terms of the influence of increasingly electron-rich pendants in raising the HOMO level, whilst the LUMO is largely unaffected (Fig. 21) [80]. In the case of the most electron-rich amine substituents (e.g. R = –C₆H₄–NMe₂), however, the emissive excited state is stabilised more than anticipated on the basis of the oxidation potential, an observation that has been attributed to a switch to an ICT-type state, involving a high degree of charge transfer, and manifest through the pronounced solvatochromism observed in emission [80, 81]. Reversible switching between the localised and charge-transfer states can be induced by protonation, and the effect can be extrapolated to azacrown pendants that bind divalent metal ions [81].

Fig. 21

Correlation of the emission energy E _em of complexes of type 47 (Z = H), carrying the aryl substituents indicated at position Y, with the oxidation peak potential measured by cyclic voltammetry in CH₂Cl₂ [80]

Whilst the above strategy of introducing electron-rich pendants successfully lowers the excited state energies, scope for shifting to the blue through modification at this position is more limited; an electron-withdrawing ester group leads to a shift of 10 nm. However, further blue shifts have been accomplished by introducing electron-donating substituents into the pyridyl rings, particularly the 4-position, which raise the LUMO energy [82]. Another way of achieving the same effect is to change from pyridine to pyrazole rings, which are poorer π-acceptors. Thus, the emission of complex 48 is blue-shifted by 1,700 cm⁻¹ compared to that of 47, though unfortunately accompanied by a substantial decrease in the emission quantum yield [83].

As for the ppy-based systems discussed in Sect. 6.2, the majority of the N^C^N-coordinated complexes display very efficient excimer formation, and the excimers themselves are remarkably highly emissive (Φ ∼ 0.3). By varying the dopant concentration within the host emissive layer, the ratio of monomeric (blue) to excimeric (red) emission is varied, and the observed light that the device produces is the net combination. At a doping level of 15% in a CBP host, a device has been obtained that emits with CIE coordinates of 0.337, 0.384, close to white light, and with a quantum efficiency of 18.1%, making it one of the most efficient WOLEDs reported hitherto [84].

In fact, the camel-shaped spectrum leads to a trough of low emission intensity between the monomer and excimer regions, making the coverage across the visible less uniform than might be desirable. Kalinowski et al. have found that this problem can be overcome by ‘filling in’ this region with emission from an exciplex [85, 86]. The concept is illustrated in Fig. 22. In the emitting region, a starburst amine hole-transporting bathophenanthroline (m-MTDATA) acts as an electron donor (D) to the Pt complex 47 (Y = –CO₂Me) as an electron acceptor (A), mixed in a 1:1 ratio. Monomer phosphor triplets, ³A*, their corresponding excimers ³(AA)* formed upon encounter with ground state A, and excited heterodimers, i.e. exciplexes, ³(DA)*, are generated throughout the emissive layer. A colour rendering index of 90 with an external quantum efficiency of 6.5% photons/electron has been attained in this way [85]. The phenomenon of electro-photoluminescence in organics has also been demonstrated for the first time recently using 47 (R = –CO₂Me) [87].

Fig. 22

Generation mechanisms of white light in an OLED based on a hole transporting material (m-MTDATA) acting as an electron donor (D) to an electron acceptor (A) molecule of an organic phosphor (47 R = CO₂Me) mixed in an emissive layer, D:A. The monomer phosphor triplets (³A*), their combination with ground state phosphor acceptor molecules [triplet excimers, ³(AA)*], and excited hetero-dimer [³(DA)*] are generated throughout the emissive layer leading to white light with colour rendering indices (CRI) up to 90 [85]

These highly emissive complexes have also been applied in sensory applications. Although quenched by O₂, they are not quenched as efficiently as longer-lived systems such as platinum porphyrins. Thus, by combining a thin layer of green-emitting Pt(N^C^N) with a second layer of red-emitting platinum porphyrin in ethyl cellulose, a traffic light response system to O₂ has been developed, in which the system emits red light in the absence of O₂, turning orange and then green as pO₂ increases, and the porphyrin becomes quenched more than the N^C^N complex [88]. Very recently, the simplest complexes of this type have also been shown to be successfully applicable to time-resolved imaging in live cells on the microsecond timescale (Fig. 23) [89]. The parent complex is highly cell permeable, entering essentially under diffusion control, and localises to the nucleoli (Fig. 24). It is also amenable to two-photon excitation in the near-IR.

Fig. 23

Time-resolved gated emission images of CHO cells incubated with 47 (Y = Z = H). The images were recorded after 355 nm laser excitation at the time delays shown between 100 and 2,900 ns after the laser flash. The time gate used was 100 ns, exposure time 0.02 s, five accumulations per time delay. Scale bar: 50 μm [89]

Fig. 24

A two-photon excitation high-resolution emission image of live CHO cells incubated with 47 (Y = Z = H), obtained under 758 nm 180 fs excitation. Scale bar = 10 μm [89]

Meanwhile, Bruce and co-workers have appended complexes of this type with long alkyl chains, adding interesting liquid crystalline (LC) properties to the luminescence. For example, the properties of an LC state of 49, obtained by slowly cooling from the isotropic melt to 170°C then rapidly cooling to room temperature, were compared with a non-crystalline phase formed from rapid cooling of the isotropic melt. Structural analysis revealed that the LC phase was composed of a columnar phase of anti-parallel complexes which are independent of one another. The non-crystalline phase, in contrast, consists of a high degree of isotropic grain boundaries, and molecules are therefore in close contact. As a result, emission from the LC state is purely monomeric, and emission from the non-crystalline phase is purely excimeric (Fig. 25). In some cases, reversible thermally-induced switching of excimer to monomer in spin-coated thin films has also been observed [90].

Fig. 25

Photomicrographs taken between cross polarisers (top) and emission spectra (bottom) of 49 (a) fast cooled from the LC phase after the texture is developed, and (b) fast cooled from the isotropic phase. Reprinted from [90] with permission from Wiley