Platinum( II ) complexes with cyclometallated 5- p -delocalized-donor-1,3-di(2-pyridyl)benzene ligands as e ﬃ cient phosphors for NIR-OLEDs †

Two new pincer proligands, namely 5-( p -( N , N -diphenylamino)phenylethynyl)-1,3-di(2-pyridyl)benzene ( HL 1 ) and trans -5-( p -( N , N -diphenylamino)styryl-1,3-di(2-pyridyl)benzene ( HL 2 ) were prepared together with their N^C^N-coordinated cyclometallated platinum( II ) complexes PtL 1 X (X ¼ Cl, NCS) and PtL 2 Cl . Both ligands are intensely luminescent in solution (quantum yields > 0.8). PtL 1 X complexes display high quantum yields in solution whereas that of PtL 2 Cl is very low due to the ease with which trans to cis isomerisation of the diphenylaminostyryl C ] C bond occurs. Distinct sets of emission bands attributable to the cis and trans forms are observable in glass at 77 K, the assignments being supported by TD-DFT calculations. Organic light-emitting diodes (OLEDs) have been prepared using the new compounds as phosphorescent emitters. Remarkably, despite the inferior quantum yield of PtL 2 Cl in solution, the best electroluminescence quantum e ﬃ ciencies are obtained with this complex, which emerges as an excellent candidate for the preparation of NIR-OLEDs.


Introduction
There has been a growing interest in the design of luminescent transition metal complexes as phosphors for organic lightemitting devices (OLEDs) over the past decade. [1][2][3] In OLEDs, light emission arises from the radiative deactivation of electronic excited states that are formed by recombination of charge carriersi.e. electrons and holesinjected from the electrodes. Because phosphorescent emitters doped into uorescent host materials can potentially harvest both singlet and triplet excitons upon electron-hole recombination, their use in place of uorescent compounds may potentially improve the luminous efficiency. Complexes of third-row transition metal ions are particularly suitable for this purpose, since the high spin-orbit coupling constant and high intersystem crossing associated with them efficiently promote emission from the otherwise wasted triplet states, which represent up to 75% of the excited states formed upon charge-recombination in an electroluminescent device. [1][2][3] Though the eld has to date been dominated by complexes of iridium(III), which are already used in many devices, platinum(II) complexes attract increasing interest. Part of the motivation for turning to platinum(II) lies in the propensity of square-planar d 8 complexes to undergo face-to-face bimolecular interactions, which may lead to excimeric or aggregate emissions that are not normally possible in d 6 complexes. 4 Several families of Pt(II) complexes have been discovered that are brightly luminescent in solution at room temperature. 4,5 A convenient strategy to improve luminescence is to make use of cyclometallating ligands, whose strong ligand elds tend to favour emission efficiencies, as they raise the energies of otherwise deactivating metal-centred states, making them thermally inaccessible and thus reducing non-radiative decay pathways. Meanwhile, tridentate ligands have been found to offer an advantage over bidentate ligands in that they impart higher rigidity on the complex, inhibiting distortion and reducing non-radiative decay. 6,7 Pt(II) complexes with tridentate ligands based on cyclometallated 1,3-di(2-pyridyl)benzene (dpyb), 8 which offer the metal ion an N^C^N coordination environment, are amongst the brightest Pt-based emitters in solution at room temperature. 9 This structural motif has also been found to lead to signicant second-order nonlinear optical properties. 10 Remarkably, the emission color of OLEDs based on these complexes can be easily tuned, for example, from blue to red by increasing the donor ability of the substituent at the central 5-position of the cyclometallating ring. 11 On the other hand, OLEDs that emit in the near-infrared (NIR) region represent an intrinsically challenging target, owing to the tendency of non-radiative decay processes to increase as the excited-state energy decreases 12 and, simultaneously, of the radiative decay constants to decrease. Two classes of NIRemitting OLEDs based on metal complexes are known. The rst utilizes lanthanide cations as the emitting centres with emission around 1000 nm but with a very low external electroluminescence quantum efficiency (EL QE). 13 The second class uses dblock metal complexes with highly conjugated ligands, characterized by higher EL QE but much shorter l max values. For example, efficient phosphorescent NIR-OLEDs based on Ptporphyrins have been reported recently, with emission maxima at around 770 nm. 14 The challenge is now to shi the l max of transition metal-based OLEDs to lower energy, whilst maintaining a high EL QE efficiency.
One way to tune emission towards the red and NIR is to make use of bimolecular excited states; for example, excimer or aggregate states of square-planar Pt(II) complexes that undergo energy-minimising face-to-face interactions. 4m,15 Recently some of us found that the complex PtL mes NCS {L mes ¼ 5-mesityl-1,3-di(2pyridyl)-benzene} allows the preparation of OLEDs that emit squarely in the NIR region (l max ¼ 855 nm), through the formation of such aggregate species and their efficient emission. 16 In this paper, we describe the effect of the incorporation of p-delocalized ArC]C-and ArC^C-substituents at the central 5-position of the phenyl ring of dpyb. The absorption, photoluminescence, and electroluminescence properties of the N^C^N-cyclometallated Pt(II) complexes are investigated, and we show how one of these novel compounds is a good candidate for the preparation of efficient NIR OLEDs.

General comments
Solvents were dried by standard procedures: THF was freshly distilled from Na/benzophenone under a nitrogen atmosphere, N,N-dimethylformamide (DMF) was dried over activated molecular sieves and triethylamine (Et 3 N) was freshly distilled over KOH. All reagents were purchased from Sigma-Aldrich and were used without further purication. Reactions requiring anhydrous or oxygen-free conditions were performed under nitrogen. Thin layer chromatography (TLC) was carried out with pre-coated Merck F254 silica gel plates. Flash chromatography (FC) was carried out with Macherey-Nagel silica gel 60 (230-400 mesh). 1

Photophysical measurements
Absorption spectra were recorded for solutions in dichloromethane within 1 cm pathlength quartz cuvettes using a Biotek Instruments XS spectrometer. Luminescence spectra were recorded using a FluoroMax-2 spectrouorimeter equipped with an R928 photomultiplier tube. Spectra were corrected for the wavelength dependence of the detector and emission grating. Quantum yields were determined using appropriate standards. For the proligands, a solution of quinine sulfate in 1 M H 2 SO 4 (aq.) was used (F ¼ 0.548). 19 9a The luminescence lifetimes of the complexes were measured by time-correlated single-photon counting, following excitation with a pulsed laser diode at 374 nm or 405 nm. The emitted light was detected at 90 using a Peltier-cooled R928 photomultiplier tube aer passage through a monochromator. 21

Density functional theory calculations
Calculations were performed using the Gaussian 09 suite of programmes 22 with the PBE0 functional. 23a The LANL2DZ basis set was used for Pt(II), with the inner core electrons replaced by a relativistic core potential, and the all-electron cc-PVDZ basis set was used for the ligands. A polarised continuum model (PCM) was used for the solvent dichloromethane. Geometries were fully optimised without symmetry constraints, and the triplet state geometries were calculated directly by minimisation of the SCF triplet state. Harmonic vibrational wavenumber calculations were performed to conrm that the structures obtained correspond to minima of the potential energy surface. Timedependent calculations were carried out on the optimised structures to determine the relevant transitions and for the generation of density difference plots. The spectral simulations were performed by convolution using a Gaussian-shape function of 0.6 eV full-width at half-maximum, as described elsewhere, 23b using the ten lowest-energy spin-allowed transitions.

Procedure for OLED fabrication and assessment
OLEDs were fabricated by growing a sequence of thin layers on clean glass substrates pre-coated with a layer of indium tin oxide (ITO), 120 nm thick, with a sheet resistance of 20 U per square. A 2 nm-thick hole-injecting layer of Mo 2 O x was deposited on top of the ITO by thermal evaporation under high vacuum of $10 À6 hPa. All remaining organic layers were deposited in succession by thermal evaporation under high vacuum, followed by thermal evaporation of the cathode layer consisting of 0.5 nm thick LiF and a 100 nm thick aluminium cap. The emitting layer (EML) was evaporated by co-deposition of PtL 2 Cl and 4,4 0 ,4 00 -tris(N-carbazolyl)triphenylamine (TCTA) or 4,4-N,N 0 -dicarbazolyl-1,1 0 -biphenyl (CBP) to form a 30 nm-thick blend lm (5 wt% Pt complex : 95 wt% TCTA or CBP), or by single deposition of the Pt(II) complex only, to form a 15 nm neat lm. The current-voltage characteristics were measured with a Keithley Source-Measure unit, model 236, under continuous operation mode, while the light output power was measured with an EG&G power meter, and electroluminescence (EL) spectra recorded with a StellarNet spectroradiometer. All measurements were carried out at room temperature under an argon atmosphere and were reproduced for many runs, excluding chemical reaction with oxygen or moisture. The performance of the emissive layer was optimized by locating the EML between exciton-blocking layers of TCTA (or CBP) (80 nm thick) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) (25 nm thick), the latter acting also as an electron-transporting and hole-blocking layer.
Complexes PtL 1 Cl and PtL 2 Cl were prepared by reaction of K 2 PtCl 4 with HL 1 and HL 2 respectively, in a AcOH-H 2 O (9 : 1 v/v) mixture placed in a microwave reactor at 160 C for 45 min, as previously described for other Pt(II) complexes. 18 The PtL 1 Cl complex was readily converted into PtL 1 NCS upon treatment with sodium thiocyanate in methanol-dichloromethane at room temperature. The two new ligands and their Pt(II) complexes were fully characterized by elemental analysis, mass spectrometry and NMR spectroscopy. 1 H NMR spectroscopy conrms that the E (trans) conguration of the C]C double bond of HL 2 is retained in the complex PtL 2 Cl.
Interestingly it turned out that complex PtL 2 Cl, in solution in CH 2 Cl 2 at room temperature, readily isomerizes to give the cis isomer upon exposure to ambient light, as revealed by the evolution of the 1 H NMR spectrum (data are provided in Fig. S1 of the ESI †). The cis isomer reconverts into the thermodynamically more stable trans isomer in the dark at room temperature (Scheme 2). In contrast, no cis isomer is formed when a solid sample of PtL 2 Cl is le under solar light at room temperature for at least a few months.

Photophysical properties in solution
Absorption and photoluminescence data of the uncoordinated proligands and of the three Pt(II) complexes are presented in Table 1.
Proligands HL 1 and HL 2 . The absorption spectra of the proligands (Fig. 1) show very intense bands in the UV region (l < 400 nm), due to spin-allowed 1 p-p* transitions. Both compounds are intensely uorescent in solution at room temperature ( Fig. 1): HL 1 emits in the blue region of the spectrum (l max ¼ 430 nm) with a luminescence quantum yield F of 0.82, whilst the emission of HL 2 is blue-green, being somewhat red-shied compared to that of HL 1 (l max ¼ 459 nm, F ¼ 0.92). At 77 K, HL 1 shows, in addition to the uorescence band, a set of structured phosphorescence bands in the 500-600 mm region, characterised by a very long lifetime approaching 1 s. In contrast, no phosphorescence is detectable for HL 2 .
Platinum complexes. The UV-visible absorption spectra of the new complexes PtL 1 Cl and PtL 1 NCS in dichloromethane solution at room temperature ( Fig. 2 and S2 †) show intense bands at l < 300 nm and another set of bands in the 330-400 nm region that are only a little less intense than the higher energy ones. Indeed, the spectra are fairly similar to those of HL 1 . They differ from those normally displayed by simple cyclometallated Pt(II) complexes with arylpyridine ligands, such as Pt(dpyb)Cl (dpyb ¼ cyclometallated 1,3-di(2-pyridyl)benzene), in that the lower-energy bands are so intense.
Typically in complexes such as Pt(dpyb)Cl, bands in the 350-400 nm region have 3 values of around 7000 M À1 cm À1 , with no counterparts in the proligands, assigned to metal-to-ligand and intra-ligand charge-transfer transitions that are introduced upon cyclometallation. 5,21 Such transitions will necessarily be present in the current complexes, but they are evidently superimposed on intense ligand-centred transitions. This leads to unusually high 3 values in this region, around 3-4 times higher than in Pt(dpyb)Cl.
Both PtL 1 Cl and PtL 1 NCS complexes are intensely luminescent in dilute solution at room temperature, displaying vibrationally structured emission spectra with maxima in the green region at 496 nm and luminescence quantum yields of around 0.25 (Table 1 and Fig. 2; spectra for PtL 1 NCS are shown in Fig. S2 in the ESI †). The prole of the spectra, attributed to a primarily ligand-centred 3 p-p* state, is in each case similar to that of the parent complex Pt(dpyb)Cl, as are the luminescence lifetimes of around 8 ms. 9 Like the parent complex, PtL 1 Cl and PtL 1 NCS readily form excimers at elevated concentration in solution, which emit in the red region of the spectrum. Interestingly, although the change of the monodentate ligand from Cl À to NCS À has no signicant inuence on the emission wavelength of the monomeric complex, the excimer emission of PtL 1 NCS is somewhat blue-shied relative to that of PtL 1 Cl (l max ¼ 655 and 680 nm, respectively). The formation of the excimer is accompanied by the expected decrease in relative intensity of the monomer bands, whilst the luminescence lifetime decreases. The gradients of the plots of the observed radiative rate constant (¼ 1/s) versus concentration provide an indication of the propensity of such complexes to form excimers. The values of 1.9 Â 10 9 and 3.6 Â 10 9 M À1 s À1 are of the same order of magnitude as those found for 5-aryl-substituted derivatives of the parent Pt(dpyb)Cl. 21   The behaviour of PtL 2 Cl, the complex incorporating the styryl substituent, is more complicated. In dichloromethane solution at 298 K, its absorption spectrum shows signicant absorption to somewhat longer wavelengths than the alkynyl complexes PtL 1 X (Fig. 3 and Table 1), but the spectrum rapidly changes in daylight or upon irradiation in the near-UV, losing the long-wavelength absorption tail. A change in the absorption spectrum upon irradiation is consistent with the light-activated trans to cis isomerisation observed by 1 H NMR spectroscopy described above. Around 7-10 minutes with a standard laboratory hand-held UV lamp (6 W, l ¼ 365 nm) is sufficient to reach a photostationary state. Apparently, then, the lowestenergy spin-allowed transitions of the trans form of the complex must be lower in energy than those of the cis form.
This tentative conclusion from the experimental data is supported by the results of time-dependent density functional theory (TD-DFT) calculations, carried out as described in the Experimental section. Examination of the 20 lowest-energy spin-allowed transitions at the ground-state geometry reveals that the cis form is indeed predicted to be blue-shied relative to the trans (the simulated spectra using the ten lowest-energy singlet transitions are provided in Fig. S3 †). The density difference plots for the lowest-energy singlet states show a signicant degree of intraligand charge-transfer character from the styryl pendant to the dipyridylbenzene moiety (see Fig. S4 in the ESI †).
The photoluminescence also changes upon irradiation. The main observations can be summarised as follows: (i) Before irradiation, the complex shows weak green phosphorescence in solution at room temperature when excited at wavelengths less than about 430 nm (Fig. 4). The structured emission prole, with the 0-0 vibrational band highest in intensity, is similar to that of Pt(dpyb)Cl. 9,21a (ii) Excitation into the long-wavelength absorption tail at l > 450 nm gives no emission. Indeed, it can be seen that the excitation spectrum registered at l ¼ 530 nm lacks the lowenergy tail that was observed in the absorption spectrum prior to irradiation.
(iii) Aer irradiation for 7 min with the UV lamp, the emission in the green region substantially increases in intensity (Fig. 4).
(iv) At 77 K in frozen glass (diethyl ether-iso-pentaneethanol), the emission spectrum displays a set of vibrationally structured bands in the range 600-800 nm. Following irradiation (in solution at room temperature), and re-recording of the spectrum at 77 K, a new set of bands appears in the range 480-600 nm, the typical region for Pt(dpyb)Cl and its simple 5-alkyl derivatives, 9,21a and the lower-energy bands lose intensity (Fig. 5).
We interpret these observations through the following assignments to the emission bands: (A) The higher-energy emission in the 480-600 nm region can be attributed to the cis isomer of PtL 2 Cl, which emitsboth at room temperature and at 77 Kmuch like Pt(dpyb)Cl and other derivatives with simple substituents in the 5-position of the aryl ring (e.g., PtL 1 Cl).
(B) The lower-energy bands in the 600-800 nm regionwhich are observed at 77 K onlycan be assigned to the trans isomer, which shows no emission at room temperature in solution.
This assignment accounts for (i) the lack of emission in solution at room temperature upon excitation at l > 450 nm, where the cis form does not absorb; (ii) the appearance of higher-energy bands (cis form) in the 77 K spectrum aer irradiation and (iii) the increase in the room temperature emission intensity upon irradiation, as the cis form builds up. The fact that there is some emission from solution even before Fig. 3 Absorption spectra of PtL 2 Cl in CH 2 Cl 2 at 298 K before (red) and after (blue) irradiation with a UV lamp for 7 minutes. Fig. 4 Emission spectra of PtL 2 Cl in CH 2 Cl 2 at 298 K before and after irradiation with a UV lamp for 7 minutes (l ex ¼ 415 nm). deliberate irradiation is probably due to the fact that isomerisation will occur as soon as the sample is exposed to the excitation source required to record the emission spectrum, and to the inevitable presence of some cis isomer formed as the sample is prepared. The observed low luminescence quantum yield observed for PtL 2 Cl in solution even in the cis form may be due to the competitive formation of a non-emissive excited state with a half-twisted conformation of the C]C unit. Similarly poor quantum yields were previously observed for a series of styryl-appended Pt(ppy)(acac) complexes, attributed to such a process. 24 TD-DFT calculations are again informative in helping to understand the differing behaviour of the cis and trans forms. Fig. 6 shows the density difference plots for the lowest-energy triplet (T 1 ) excited states of the two isomers, calculated at their triplet-state geometries. It can be seen that the triplet excited state of the cis form is largely located on the Pt(N^C^N)Cl moiety, with little involvement of the pendant. Indeed, the orbital parentage of the excited state is similar to that of the parent complex Pt(dpyb)Cl obtained using comparable calculations, 21b consistent with the observation that the emission energy and the spectral prole of cis-PtL 2 Cl are similar to those of the parent. In contrast, the density difference plot for the triplet state of trans-PtL 2 Cl spans the metallated aryl ring, the C]C bond, and the pendent aniline unit (Fig. 6). There is a more extended conjugated unit, consistent with the unusually low emission energy compared to Pt(dpyb)Cl. Indeed, it is notable that the emission energy is not dissimilar to that of E-stilbene phosphorescence (l max ¼ 580 nm). 25 It is also apparent from the density difference plot that metal orbitals seem to play a more minor role in the excited state compared to the cis. This might account for the longer lifetime of the emission bands of the trans compared to the cis form at 77 K (13 and 6.7 ms respectively, Table 1), since it is the inuence of the metal that promotes the formally forbidden T 1 / S 0 phosphorescence process.

OLED characterization
The three new complexes PtL 1 Cl, PtL 1 NCS, and PtL 2 Cl were examined as phosphors for OLEDs, in the form of neat lms, where the intermolecular interactions might be expected to become important. OLEDs were fabricated by growing a sequence of thin layers on glass substrates pre-coated with indium tin oxide (ITO), transparent to the light generated in the emitting layer (EML) (see Experimental section for details). Holes were injected from the ITO/Mo 2 O x anode, and passed through a 80 nm thick hole-transporting layer composed of TCTA. They recombine in the EML (15 nm thick layer of the pure Pt(II) complex) with electrons injected from an Al/LiF cathode and transported through a 25 nm layer of TPBi (1,3,5-tris(Nphenyl-benzimidazol-2-yl)benzene). Fig. 7 and 8 show the plots of luminance and electroluminescence intensity, respectively, versus voltage for the OLEDs prepared with the neat platinum(II) complexes as emitting layers, whilst Fig. 9 shows the electroluminescence quantum efficiencies (EL QE) versus the electric current density. Clearly, although the three complexes have a similar brightness and electroluminescence intensity, PtL 2 Cl is the best candidate for the preparation of OLEDs, since its device is characterized by a high electroluminescence quantum efficiency (QE). The much lower QE of the devices incorporating PtL 1 Cl and PtL 1 NCS can be attributed to some degradation, as conrmed by the unstable electroluminescence spectra of the related OLEDs (see ESI †).
The electroluminescence spectrum of the OLED based on neat PtL 2 Cl as an emitting layer is stable (all measurements were reproduced for many runs, excluding any irreversible chemical and morphological changes in the devices) and it exhibits a very broad, low-energy, structureless band around Fig. 6 Density difference plots for the lowest triplet excitations of the cis (a) and trans (b) forms of PtL 2 Cl calculated at the optimised T 1 geometry, using PBE0 with the PCM model for CH 2 Cl 2 . Yellow and purple represent zones of depletion and augmentation of electron density in the T 1 excited state versus the S 0 ground state.  This spectrum is very different from that typical of devices based on other reported platinum(II) derivatives with cyclometallated 1,3-di(2-pyridyl)benzene ligands, which exhibit excimer-like emission from neat lms, with l max around 685 nm. 5c,d,g Such an EL spectrum, drastically shied into the NIR spectral region, is of particular interest for these kinds of complexes. A comparably low-energy emission has been previously observed only for PtL mes NCS, as mentioned in the Introduction. 16 As in the latter complex, the metal-metal interactions within the neat lm apparently lead to the formation of lowenergy aggregates that emit in the NIR region. The NIR OLED shows the best performance with an EL intensity of about 10 mW cm À2 at 9 V and a maximum QE of 1.2% ph/e at a current density of 10 mA cm À2 . It is worth noting that the QE of this device remains fairly constant over a wide range of current density from 10 À3 to 400 mA cm À2 .
On the other hand, the EL spectra of the OLEDs with 5 wt% of PtL 2 Cl in TCTA or CBP are quite similar to one another and are clearly characterized by emission from the monomolecular excited states of the trans isomer around 600 nm together with a contribution of the TCTA or CBP emission bands (Fig. 10, blue and green lines respectively). The CIE coordinates are x ¼ 0.302, y ¼ 0.203 and x ¼ 0.537, y ¼ 0.203 for lms in TCTA and CBP, respectively. The plots of luminescence versus applied voltage and external quantum efficiencies versus electric current density are shown in the ESI. †

Conclusions
In summary, we have prepared and characterized two new highly luminescent N^C^N pincer ligands incorporating triple ArC^C-(HL 1 ) or double ArCH]CH-(HL 2 ) bonds at the 5-position of the central aryl ring, together with their cyclometallated platinum(II) complexes PtL 1 X (X ¼ Cl, NCS) and PtL 2 Cl. Complexes PtL 1 X display high photoluminescence quantum yields in solution. They also have much higher molar absorptivities than the parent complex Pt(dpyb)Cl and its simple 5-aryl derivatives, the 3 values in the 350-400 nm region being around 3-4 times greater. In the eld of luminescent probes, for example in bioimaging and sensing, it is currently popular to refer to a "brightness" index, which is the product of the extinction coefficient and quantum yield, reecting the importance of both quantities in determining suitability for practical use. 26 In the case of the new complexes PtL 1 X, although the quantum yield is somewhat compromised compared to the parent (factor of 2), it is clear that their brightness would be signicantly superior owing to the bene-cial effect of the substituent on the 3 values.
Contrary to the C^C-substituted complexes, the photoluminescence quantum yield of PtL 2 Cl is very low in solution at room temperature, due to competitive photoinduced trans-to-cis isomerisation of the diphenyl-aminostyryl C]C bond.
OLEDs have been prepared using the new compounds as phosphorescent emitters. Remarkably, the best electroluminescence quantum efficiencies are obtained with PtL 2 Cl. The isomerisation process observed under photoexcitation evidently does not occur in the emissive layer under electrically driven conditions. This study highlights how photoluminescence performance in solution is not necessarily a good guide to electroluminescence efficiency in a device.   PtL 2 Cl appears as an excellent candidate for the preparation of NIR-OLEDs. The electroluminescence intensity of the studied NIR-OLED is three times higher than the previously reported NIR-OLED based on a neat lm of PtL mes NCS, 16 whilst the maximum external quantum efficiency is enhanced by 20%. The stability and the limited efficiency-roll-off of the NIR-OLED at high current density are good characteristics for applications requiring high NIR intensity. NIR-emitting systems have diverse potential applications, including communications and night vision-readable displays, as well as offering superior biocompatibility for medical systems since biological tissue is most transparent to light in the NIR.