Red-emitting cyclometalated platinum(II) complexes with imidazolyl phenanthrolines: Synthesis and photophysical properties

Accepted Manuscript Title: Red-emitting cyclometalated platinum(II) complexes with imidazolyl phenanthrolines: Synthesis and photophysical properties Authors: Ankita Sarkar, Venkata N.K.B. Adusumalli, Parna Gupta PII: DOI: Reference:

S1010-6030(18)30308-3 https://doi.org/10.1016/j.jphotochem.2018.04.045 JPC 11260

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

12-3-2018 13-4-2018 25-4-2018

Please cite this article as: Ankita Sarkar, Venkata N.K.B.Adusumalli, Parna Gupta, Red-emitting cyclometalated platinum(II) complexes with imidazolyl phenanthrolines: Synthesis and photophysical properties, Journal of Photochemistry and Photobiology A: Chemistry https://doi.org/10.1016/j.jphotochem.2018.04.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Red-emitting cyclometalated platinum(II) complexes with imidazolyl phenanthrolines: Synthesis and photophysical properties Ankita Sarkar, Venkata N. K. B. Adusumalli and Parna Gupta* Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur - 741 246,

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West Bengal, India E-mail: [email protected]

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Graphical abstract

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Highlight

Synthesis, electronic spectral and electrochemical characterization of three new

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cyclometalated platinum-polypyridyl complexes. Significant Pt….Pt and π - π interaction in the solid state involving 3MMLCT state.

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Solid state emission is significant for one of the complexes and the emission spectra

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with the commercial LED (6V) shows red emission.

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Excited-state properties are corroborated by static and time-dependent density-

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functional theory.

Abstract: Three cyclometalated platinum(II) complexes with substituted imidazolyl phenanthrolines L1 - L3 have been synthesized and characterized using spectral and electrochemical techniques. The effect of substituent (in the appended aromatic ring to the imidazolyl moiety) on the electronic feature and redox potential of the cyclometalated 1

platinum(II) complexes of general formula [Pt(ppy)(L1-L3)]Cl (1 - 3) are discussed. Complexes 1 - 3 are luminescent in the solution and solid state. Complex 1, with two trifluoromethyl moieties present in the ligand shows a higher degree of Pt II….PtII interaction than the ligand incorporated with N,N-dimethyl moiety. The excimer formation through andd Pt...Pt) interactions is evident and the degree of excimer formation is in the order 123. Excited-state properties are corroborated by static and time-dependent densityfunctional theory.

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Keywords: Cyclometalation • Platinum • Phenanthroline • Luminescence • Electrochemistry • TDDFT

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1. Introduction

Luminescent cyclometalated complexes of heavy transition metals have been exhaustively studied for their rich phototophysical properties1 and wide application as electroluminescent

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materials,2 chemosensors,3 photocatalysts,4 probes for bioimaging,5 singlet-oxygen generator6

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and photodynamic therapeutic agent.7 However, the iridium(III) and platinum(II) complexes gained more prominence as luminescent materials due to their ability to harvest both the

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singlet and triplet excitons, sometimes resulting in a theoretical 100% electron-to-photon

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conversion efficiency. Population of triplet excited states builds up in these substances by absorption of visible light and facile intersystem crossing due to high spin-orbit coupling of

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the heavy transition metal (Ir: ζ 3909 cm−1; Pt: ζ = 4481 cm−1).8 The quest for stable complexes that exhibit highly efficient triplet emissions, relatively long excited-state lifetimes and tunable colours has been a constant in this field, associated with the need to

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improve the performance and reliability of the specific applications. Square planar platinum complexes with diverse cyclometalating as well as ancillary

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ligands possess excellent structural flexibility with the ability to form aggregate through stacking interaction.9 In doing so, the ground state and excited state properties of platinum complexes alter significantly. They have been extensively studied for several decades

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because of their unique color and photophysical properties attributed to the various stacking interactions (e.g., Pt…Pt and/or π−π-stacking interactions).1b,9a,10 The isolated square-planar Pt(II) complexes show transition typically involving a mixture of ligand-centered (LC) and metal-to-ligand charge transfer (MLCT). However, for the dimeric/aggregated compound the lowest energy transition may best be described as metal−metal-to-ligand charge transfer (MMLCT). It may involve d-π interactions between one filled 5dz2 Pt(II) orbital and a πorbital from a proximal molecule, d-d interactions from the direct interaction of the 5dz2 2

Pt(II) orbitals, or a combination. The splitting of the z-oriented 5dz2 (occupied) and 6pz (unoccupied) atomic orbitals give filled dσ and dσ* in addition to unfilled pσ and pσ* molecular orbitals. The overlap of the two filled 5dz2 orbitals directed along the Pt-Pt axis and the two unfilled 6pz orbitals in the planar Pt(II) complexes allows the intermolecular metal-metal (PtII....PtII) contact. This phenomenon mostly accounts for the room temperature solid state emission, the prerequisite for their applications in the organic light emitting diode (OLED) based display. Solid-state emission is important not only for the development of

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future light-emitting devices but also in regard to fundamental research for new functions of

molecular aggregations. High-density integration is a straightforward concept to increase the

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brightness of light-emitting materials; however, most materials that exhibit intense emission under low density environments cannot maintain their emission efficiencies in the crystalline state due to the inevitable energy loss by nonradiative pathways through intermolecular connection.

complexes of general formula [Pt(ppy)(Ln)]Cl, where n=1-3, containing

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platinum(II)

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In this study, we investigated the photophysical behaviour of three cyclometalated

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substituted imidazolyl phenanthroline ligand (L1 - L3). Though cyclometalated platinum(II) with polypyridyl ancillary ligand are amply present, complexes with substituted

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phenanthroline ligand have not been explored much.11 The complexes 1 - 3 show phosphorescence in the solution and the solid state at room temperature. Insight on the

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photophysical studies reveal that  as well as Pt….Pt stacking interaction is present for the reported complexes. The platinum centres interact more strongly in the complexes 1 and 2

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than in complex 3. Theoretical calculations by the DFT/TDDFT method support our understanding of the electronic transitions. The nature of the substituted group indeed

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affected the emission and it is expected that the predicted relationship between the structure and property may give more insights into the core issues regarding the design and synthesis of new platinum (II) phosphors. Result and Discussion

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The ligand system has immense influence on the photophysical properties of the transition metal complexes.2a,

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The photophysical properties of the platinum (II) complexes are

dependent on the cyclometalated(C^N) as well as the ancillary ligands (N^N). This is because the energies of the HOMOs and LUMOs of these complexes can be independently modulated as a function of appropriate substituent modification of the C^N and N^N ligands. In the present study 2-phenylpyridine is the sole cyclometalated ligand. We choose imidazolyl-

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phenanthroline as the basic ancillary ligand framework and the appended phenyl moiety to the imidazolyl ring contains bis-3,5-CF3 and 4-NMe2 in L1 and L3 respectively; L2, contains only the phenyl ring as depicted in Scheme 1.13 The variation in the substituent will alter the electronic effect imparted by the ligand. This may help to tune the emission wavelength. In addition, the steric restriction imposed by them may also affect the stacking interaction and thus the photophysical parameters. The ligands are synthesized by following the reported procedures14 and have been confirmed by ESI-MS and 1H NMR. The target complexes 1 - 3

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were synthesized in two steps from K2PtCl4 via the precursor [Pt(ppy)Cl]2 dimer.15 The synthesized dimer reacts with L1 - L3 in the presence of dry DMF to give 1-3 respectively.

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The composition of the complexes as [Pt(ppy)(L1 - L3]Cl were confirmed by ESIMS and NMR (Figure S1, S2). The complexes are sparingly soluble and this is reflected in their 1H NMR spectra. Despite several attempts we failed to obtain good quality crystals.

Thus, the molecular structure without the counter-ion (Cl-) elucidated through geometry

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optimization is shown in Figure 1. The bond lengths and bond angles matches quite well

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with the earlier reported crystal structure. [16]

Photophysical Characterization: The electronic spectra of the ligands L1 - L3 in

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dimethylformamide (DMF) reveals intense * based absorption peaks in the region 270 360 nm, with a weak shoulder at 391 nm for L3. The ligands L1 and L2 emit in the violet-

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blue region, whereas, L3 emits at green region (Figure S3). Emission spectra of L1 and L3 have mirror symmetries with small shift from the corresponding absorption spectra. (Figure

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S3) Therefore, emissions of proligands L1 and L3 are believed to represent the S1  S0 radiative transition which is typical for organic molecules without heavy atoms.

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The photophysical properties of 1 - 3 were studied both in the solution as well as in the solid state. The complexes are soluble in DMF and partly in DMSO. We therefore recorded the electronic spectra in DMF and the pertinent data are presented in Table 1. The complexes 1 - 3 (Conc.: 10-5 M) absorb in the range 300 - 550 nm in the solution state. The

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intense absorptions observed in the ultraviolet region (300 - 370 nm) originate from the localized ligand centered * transition. These bands appear energetically very close to the absorption bands of the respective ligands. The absorption in the 400-550 nm range can be assigned to the spin-allowed singlet dπ(Pt)π*(L) metal-to-ligand charge transfer (1MLCT) or (1LMCT) in combination with the intra-ligand π π* transition. The long-wavelength tail of the complexes, the absorption, partly attributed to the triplet metal-to-ligand charge 4

transfer (3MLCT) transition. The spin-forbidden nature in the transition has become partially allowed due to the enhancement of spin-orbit coupling through the Pt(II) metal center. The position and intensity of the MLCT/* transitions are affected by the nature of the substituents present in the imidazolyl-phenanthroline ligand. The presence of electronwithdrawing CF3 substituent on the 3 and 5 position of the phenyl ring results in a blue shift of the low-energy band in complex 1 (λmax= 487 nm), while the electron-donating NMe2 group (due to intramolecular charge transfer) causes the red shift in compound 3 (λmax = 535

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nm) when compared to the unsubstituted complex 2 (λmax = 506 nm)(Figure 2). However, the

solid state absorption spectra show shift towards longer wavelength for 1 and 2; the

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absorption for the complex 3 is similar to the solution state spectra. The anomaly may be due

to the packing of the molecules in the solid state being different in the complex 3 from the rest. Therefore, depending on the ground-state Pt…Pt distance, the lowest energy transition evolves from 3MMLCT transition. We shall be discussing the solid state photophysical

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behaviour in the subsequent section in details.

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Cyclometalated platinum(II) diimine type complexes always show strong emission in the

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solution. Emission spectra of the complexes in DMF and solid state are displayed in Figure 3, and the spectral data is presented in Table 1.

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As the complexes contain 2-phenyl pyridine and the ancillary ligand comprised of conjugated system, the chance of excimer formation is quite high. To understand the

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dominance of monomer/excimer present in the solution, the concentration dependent emission spectra have been recorded. The emission of 10-6 - 10-4 M solution of the complexes

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shows strong dependence on the concentration and the excitation wavelength. At low concentration (10-6 - 10-5M), 1 - 3 show low-intensity emission maxima around 540 nm (ex =

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466nm). In addition a broad emission at 620 nm is observed for 10-5M solution of 1. Excitation spectra matches well with the absorption spectra and it unambiguously shows 1

LLCT/1MLCT (370nm) and small 3MLCT(466nm) contribution (Figure 2d). With

increase in the concentration to 10-4M, emission maxima (λex = 466nm) at 540 nm diminishes

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and all the complexes emits above 620 nm (1: 635nm, 2: 658 nm; 3: 656 nm (shoulder) Figure S4). The 10-3M solution show broad unstructured highly intense emission at 665, 657, 666 nm (λex = 466 nm) for the complexes 1 - 3 respectively (Figure S4). The emission of 103

M solution is independent of excitation wavelength in the range 460 - 530 nm. Excitation

spectra of 2 and 3 resemblance well with the absorption spectra of 10-5M solution. The absorption spectrum of the 10-3M solution (To understand the peak at 514 in excitation 5

spectra) in 400-700 nm region is recorded for 1, and it exhibits a new shoulder in the low energy region (540 nm) for 1. According to the literature,[17] the broad and relatively structured bands are attributed to the excimer and monomer emissions, respectively. The redshift in the emission energy with increase in the concentration owes to  interaction as well as Pt - Pt interaction. Thus, it was apparent that the emission and the tendency to aggregate and form excimer differ quite significantly among complexes. The bathochromic shift in the electronic spectra thus could be ascribed to excimer formation with a new MMLCT emissive state (d-π interactions). For complex 1, the emission in dilute solution at

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3

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around 620 nm is 3MLCT based, which is reflected in the solid state emission as well.9

A biexponential fit of the decay kinetics at both emission maxima and for all three concentrations was conducted to determine the decay constant of the monomer and the excimer emission. It was found that the monomer emission decays with  3.6 μs and the

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excimer emission with a   14.5 μs,   19.31 μs   4.86 μs for the 1 - 3 respectively. Thus,

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the complexes 1 and 2 are quite stable in the excimer state, whereas, the lifetime of complex

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3 is comparable with the monomeric state. The quantum yields of the complexes are moderate and comparable to similar platinum complexes (Table 1).18

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The complexes 1 - 3 show solid state emission at room temperature. The emission maxima are red shifted for 1 and 2, and blue-shifted for the 3. The emission vibronically resolves for 1 3

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at 631 and 713 nm (Table 1, Figure 4) and these two maxima corresponds to 3MLCT and MMLCT transition respectively. The increase in the lifetime and the quantum efficiency,

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due to the decreased non-radiative decay than in solution for 1 and 2, indicates the strengthening of the molecular aggregations in the solid state (Table 1). The quantum yield

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and the lifetime cannot be determined experimentally for the complex 3, with appended NMe2. The photographic images of the emission color for 1 - 3 in solid state is shown in Figure 4.

The optimization of the dimeric structures for the complexes 1 - 3, show head to head

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interaction with Pt….Pt distances for complexes 1 and 2 are 3.73Å and 3.79Å (Figure 5) respectively. However, the optimization does not lead to such closely spaced dimeric structure for complex 3. Interestingly, we could see that the molecular orientation of the molecules allows both the PtPt and  interaction in the complexes and the Pt-Pt distance of 3.7 Å allows the  interaction quite comfortably.19 To see the emission by using a combination of a 450 nm 6

commercial LED as excitation source), solution of 1 (10-4M) and glass slide coated with 1 were exposed to the LED source. The emission spectra were recorded using the commercial LED (6V) as the excitation source (Figure 6). In both the cases it shows red emission. Electrochemistry: The redox potentials of the complexes were determined by cyclic voltammetry measurements (standard Fc/Fc+) and the electrochemical data were tabulated in Table 3. The complexes 1 and 2 show only single oxidation process at 1.14V for 1 and 0.99V

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for 2. Two oxidation processes are observed for 3. The 0.17 V shift in the oxidation potential of 1 indicate additional perturbation of the electronic structure of the L1 compared

to L2 in 2. The first oxidation for these two complexes are Pt(II) centred (MLCT). The

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composition of HOMO in the complexes 1 and 2 confirm the assignment of the first oxidative

response as MLCT based (Figure S5) oxidation. However, the HOMO of complex 3 is purely ligand based and the electronic distribution show the imidazolyl-phenanthroline ligand contributes the most. Thus, it is wise to confirm first oxidation as the ligand based oxidation

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and the second oxidation as metal-based. The reduction processes are essentially ligand

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localized. All the processes are irreversible. The potentials in the excited state (Eox* and

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Ered*) for both the complexes have been estimated from the ground-state redox potentials and the energy of the excited state corresponding to the maximum of the emission spectrum at

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298 K, i.e., 634 nm (1.95 eV), 666 nm(1.86 eV) and 619 nm (1.91 eV) for 1 - 3 respectively. All the complexes are strongly oxidizing with Ered* in the range 1.47 V to 1.57 V vs Ag/AgCl

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in its excited state. The data were presented in the Table 2. Theoretical aspect: The molecular and electronic structures of the complexes were studied

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by combined density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations at the B3LYP/(6-31G**)+LANL2DZ level. Calculations correctly reproduce the near-square planar coordination of the metal centre. The calculated energy

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wavelengths of the S0 → S1 transitions, at 519, 506, and 554 nm for 1, 2 and 3, respectively, are close to the observed onsets of the absorption spectra recorded in Figure 2. An analysis of the TDDFT results (Table 3, Figure 7, 8, S5) explains the nature of the transitions and the

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data indicate spin allowed 1LC transitions from ppy/L to *ppy/*L (partially or fully) for absorptions by complexes with high ε values. The contribution of (ppy)-*(L) for complexes 1 and 2, (L)-*(L) for complex 3 along with spin allowed 1MLCT (metal-toligand charge transfer) around 415 nm are reasonably concordant with the oscillatory frequency obtained through DFT-TDDFT calculations. The transition above 500 nm mostly involves ML{(*(ppy)}CT transition for complexes 1 and 2, and L{(*(L)}MCT for 3. 7

The excited states ordering of 1 - 3 show that T1, T2 states lie below S1 for 1 and 3, which indicates that intersystem crossing principally occurs via S1T2; whereas, for 2, S1T1 transition takes place (Figure 7). Moreover, the calculated energy wavelengths of the T1 states, 540, 571 and 559 nm for 1, 2 and 3, respectively, are also in good agreement with the trend of the first vibronic peak of their phosphorescence spectra. The complexes were found to exhibit luminescence at room temperature from a 3LC state with significant metal

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character perturbed by a low-lying 1MLCT/3MLCT state.

Conclusion

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In conclusion, we have synthesized and characterized three new cyclometalated platinum(II)

complexes. The complexes were found to exhibit orange-red luminescence in DMF solution at room temperature from a 3LC state with significant metal character perturbed by a lowlying 1MLCT and 3MLCT state. They exhibit long-lived, bi-exponential excited state lifetime

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decay with the first component ranged around 3.6 μs for 1 and 2, and 4.65 μs for 3; and the

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second component contributed by excimer emission with long lifetime around 14.9 and 19.3

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μs for 1 and 2 respectively. The highly concentrated solution shows the presence of excimer formed through Pt….Pt/ π - π interaction. The excimer formation is the most pronounced for

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1. The quantum yield from monomeric state is more for complex 3 than 1and 2. Solid state emission is mostly attributed to 3MMLCT state generated through π-d/d-d interactions. All

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the complexes exhibit metal based oxidation at near 1 V and the first reduction occur around −0.36V to −0.39 V vs. SCE. The complexes show more than two reductions and they involve

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primarily * orbitals on the imidazolyl phenanthroline ligand. Thus, manipulation of the photophysical parameters can be done by changing the electronic influence imparted by the

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substituent in the imidazolyl phenanthroline ancilliary ligand. We are working on the structural modifications of the ligand system to synthesize complexes with higher quantum yield to get improved OLED emitter. Experimental Section

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The starting materials K2PtCl4, 3,5-bis(trifluoromethyl)benzaldehyde, benzaldehyde, 4(dimethylamino)benzaldehyde were purchased from Sigma-Aldrich and used without purification. Analytical grade solvents were obtained from commercial suppliers and dried by usual methods prior to use. Ligands L1 – L3 were synthesized using literature procedures.13 1

H spectra were recorded at 25 C on a JEOL ECS 400 using TMS as the internal standard.

Elemental analyses were determined on a Perkin-Elmer. Electrochemistry was done with CH 8

Instruments Model 600C Series Electrochemical Analyzer/Workstation with a potential sweep rate of 100 mV s−1. A platinum disk working electrode, a platinum wire auxiliary electrode and an aqueous Ag/AgCl were used in a three electrode configuration. Electrochemical

measurements

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made

under

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dinitrogen

atmosphere.

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electrochemical data were collected at 298 K and are uncorrected for junction potential. Mass spectra were recorded on a Q-Tof Micromass spectrometer by positive-ion mode electrospray ionization. The electronic spectra were recorded with a U-2900 spectrophotometer from

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Hitachi. The emission measurements and phosphorescence lifetime were performed with a Fluoromax 3 spectrofluorometer from Horiba Jobin Yvon. Quantum yield data reported here

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were measured relative to Ru(bpy)3Cl2 in acetonitrile (λex = 450nm, Φ = 0.059).20 The integration of the emission spectra were obtained from the Fluoromax-4 instrument from Horiba Jobin Yvon equipped with a 150W Xe lamp directly. Solid state absorbance spectra of the complexes were recorded using a Jasco V-670 spectrophotometer. To record the

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absorbance spectra of the complexes, the spectrophotometer was equipped with an integrating

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sphere. All calculations were performed in Gaussian 09. Singlet and triplet ground state

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geometries were optimised using the density functional hybrid model B3LYP together with the 631g(d,p) basis set and the Hay-Wadt-ECP (LANL2DZ), for platinum. Frontier molecular

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orbitals were computed on the singlet ground state structures while the absorption transitions were assigned by TD-DFT at the same level of theory. First 40 excitations from the optimised was used.12d, 19, 21, 22

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Synthesis of ligand

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singlet state geometry were calculated without spin restriction. For visualisation GaussView

L1: 420.4 mg (2.0 mmol) 1,10-phenanthroline-5,6-dione was dissolved in 20 ml glacial acetic acid, added 5.78 g (75.0 mmol) of sodium acetate and refluxed. To this refluxing solution 605

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mg (2.5 mmol) of 3,5-Bis(trifluoromethyl)benzaldehyde in 15 ml warm glacial acetic acid was added and refluxed for two hours. Then the resulting yellow coloured solution was cooled to 0C and neutralised with aqueous ammonia solution (pH=7). The red-orange

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precipitate was filtered, washed with water, methanol and dried in vacuum.Yield: 89.1%; 1H NMR (400 MHz, DMSO-d6) (ppm) 9.06(d, 2H, J = 16), 8.92-8.89 (b, 2H), 8.39(d, 2H, J = 12), 8.05(s, 1H), 7.85(d, 2H, J = 20). ESI-MS: m/z = 432.30; L2: The procedure was similar to that for L1, except that benzaldehyde (265 mg, 2.5 mmol) was used in place of 3,5-Bis(trifluoromethyl)benzaldehyde. Yield: 76.4%; 1H NMR (400

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MHz, DMSO-d6)  (ppm) 9.01(d, 2H, J = 8), 8.91(d, 2H, J =7.64), 8.28 (d, 2H, J = 7.60), 7.81(m, 2H, J =12.20), 7.59(t, 2H, J = 7.64), 7.50(t, 1H, J =6.88). ESI-MS: m/z = 296.33; ( L3: The procedure was similar to that for L1, except that 4-(Dimethylamino)benzaldehyde (373 mg, 2.5 mmol) was used in place of 3,5-Bis(trifluoromethyl)benzaldehyde. Yield: 82.6%; 1H NMR (400 MHz, DMSO-d6)  (ppm) 13.35(s, N-H), 8.93(s, 2H), 8.84(d, 2H, J =7.64), 8.05(d, 2H, J = 7.60), 7.75(s, 2H), 6.83(d, 2H, J = 8.40), 2.95(s, 6H). ESI-MS: m/z =

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339.39: Synthesis of complexes

1: 54 mg (0.1 mmol) [Pt(ppy)(Hppy)Cl] and 44 mg (0.1 mmol) of L1 were refluxed in 5 mL

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DMF for 18h. The resulting red precipitate is filtered and washed several times with

dichloomethane, methanol and water. Yield: 54.1%; Elemental Anal. Calc. for: C32H18ClF6N5Pt: C, 47.04; H, 2.22; N, 8.57. Found: C, 47.32; H, 2.31; N 8.45; 1H NMR (400 MHz, DMF-d7) (ppm): 9.09(d, 2H, J = 6.12), 8.96 (d, 2H, J = 6.12), 8.92 (s, 1H), 8.65 (d,

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2H, J = 5.36), 7.91(t, 4H, J = 7.76), 7.44 (d, 2H, J = 6.08), 7.37 (t, 2H, J = 6.12). ESI-MS:

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m/z = 780.81;

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2: Yield: The procedure was similar to that for 1, except L2 (30 mg, 0.1 mmol) was used in place of L1. 52.5%; Elemental Anal. Calc. for: C30H20ClN5Pt C, 52.91; H, 2.96; N, 10.28.

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Found: C, 53.10; H,2.75; N, 10.22; 1H NMR (400 MHz, DMSO-d6) (ppm): 13.57(s, N-H), 9.09(s, 2H), 8.74(t, 1H, J = 4), 8.44(d, 1H, J = 4), 8.09(d, 1H, J = 6), 8.07(d, 1H, J = 6.72),

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7.93(t, 1H, J = 6.08), 7.85(d, 2H, J = 6), 7.72(d, 1H), 7.65(t, 1H, J = 6), 7.38(t, 2H, J = 5.56), 7.27(t, 2H, J = 5.8), 7.12(t, 1H, J = 5.8), 6.97(t, 1H, J = 6.32). ESI-MS: m/z = 645.23;

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3: The procedure was similar to that for 1, except L3 (34 mg, 0.1 mmol) was used in place of L1. Yield: 51.8%; Elemental Anal. Calc. for: C32H25ClN6Pt C, 53.08; H, 3.48; N, 11.61.

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Found: C, 53.14; H, 3.79; N, 11.82; 1H NMR (400 MHz, DMSO-d6)  (ppm): 8.67(s, 2H), 8.08(d, 4H, J = 5), 7.95(s, 2H), 7.88(t, 1H, J = 5.8), 7.68 – 7.65(broad, 1H), 7.49(t, 4H, J = 5), 7.43(d, 2H, J = 5.32), 7.35(t, 2H, J = 4.04), 3.14(s, 6H). : ESI-MS: m/z = 689.39; Acknowledgement

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PG would like to thank the Department of Science and Technology, India, research grant SR/FT/CS-057/2009. Ankita thanks the DST, India for her INSPIRE fellowship and Venkata N. K. B. Adusumalli thanks UGC, India. Authors are grateful to Dr Venkataramana Mahalingam, IISER Kolkata for his help.

Authors like to thank Sk Atiur Rehman for

photographic images.

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References: [1]

a) D. A. K. Vezzu, J. C. Deaton, J. S. Jones, L. Bartolotti, C. F. Harris, A. P.

Marchetti, M. Kondakova, R. D. Pike and S. Huo, Inorg. Chem., 2010, 49, 5107 - 5119. b) A. Diez, J. Fornies, C. Larraz, E. Lalinde, J. A. Lopez, A. Martin, M. T. Moreno and V. Sicilia, Inorg. Chem., 2010, 49, 3239 - 3251. c) T. Strassner, Acc. Chem. Res., 2016, 49, 2680

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- 2689.

[2] a) J. Zhao, F. Dang, Z. Feng, B. Liu, X. Yang, Y. Wu, G. Zhou, Z. Wu and W. Y. Wong,

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Chem. Commun., 2017, 53, 7581-7584. b) S. Fuertes, A. J. Chueca, L. Arnal, A. Martin, U.

Giovanella, C. Botta and V. Sicilia, Inorg. Chem., 2017, 56, 4829 - 4839. c) B. -Y. Ren, R. D. Guo, D. -K. Zhong, C. –J. Ou, G. Xiong, X. –H. Zhao, Y. –G. Sun, M. Jurow, J. Kang, Y. Zhao, S. –B. Li, L. –X. You, L. -W. Wang, Y. Liu and W. A Huang, Inorg.

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Chem., 2017, 56, 8397 - 8407. d) X. Yang, G. Zhou and W. -Y. Wong, Chem. Soc. Rev.,

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2015, 44, 8484 - 8575.

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[3] a) D. Qiu, M. Li, Q. Zhao, H. Wang and C. Yang, Inorg. Chem., 2015, 54, 7774 - 7782. b)

Commun., 2016, 52, 3611 - 3614.

M

W. Wang, Z. Mao, M. Wang, L. -J. Liu, D. W. J. Kwong, C. -H. Leung and D. -L. Ma, Chem.

ED

[4] a) T. Kowacs, L. O'Reilly, Q. Pan, A. Huijser, P. Lang, S. Rau, W. R. Browne, M. T. Pryce and J. G. Vos, Inorg. Chem., 2016, 55, 2685 - 2690. b) J. Twilton, C. C. Le, P. Zhang,

PT

M. H. Shaw, R. W. Evans and D. W. C. MacMillan, Nat. Rev. Chem., 2017, 1, 0052. c) T. Stoll, C. E. Castillo, M. Kayanuma, M. Sandroni, C. Daniel, F. Odobel, J. Fortage and M. N.

CC E

Collomb, Coord. Chem. Rev., 2015, 304, 20 - 37. d) S. Metz and S. Bernhard Chem. Commun., 2010, 46, 7551 - 7553. [5] a) K. K. W. Lo, Acc. Chem. Res., 2015, 48, 2985 - 2995. b) J. R. Berenguer, J. G. Pichel,

A

N. Gimenez, E. Lalinde, M. T. Moreno and S. Pineiro-Hermida, Dalton Trans., 2015, 44, 18839 -18855. c) Z. Guo, W. L. Tong and M. C. W. Chan, Chem. Commun., 2014, 50, 1711 1714. d) G. U. Reddy, P. Das, S. Saha, M. Baidya, S. K. Ghosh and A. Das, Chem. Commun., 2013, 49, 255 - 257. [6] a) A. Jana, L. McKenzie, A. B. Wragg, M. Ishida, J. P. Hill, J. A. Weinstein, E. Baggaley and M. D. Ward, Chem. Eur. J., 2016, 22, 4164 - 4174. b) E. Palao, R. Sola-Llano, A. 11

Tabero, H. Manzano, A. R. Agarrabeitia, A. Villanueva, I. Lopez-Arbeloa, V. MartinezMartinez and M. J. Ortiz, Chem. Eur. J., 2017, 23, 10139 - 10147. c) N. M. Shavaleev, H. Adams, J. Best, R. Edge, S. Navaratnam and J. A. Weinstein, Inorg. Chem., 2006, 45, 9410 9415. [7] a) R. E. Doherty, I. V. Sazanovich, L. K. McKenzie, A. S. Stasheuski, R. Coyle, E. Baggaley, S. Bottomley, J. A. Weinstein and H. E. Bryant, Sci. Rep., 2016, 6, 22668. b) X.

IP T

Tian, Y. Zhu, M. Zhang, L. Luo, J. Wu, H. Zhou, L. Guan, G. Battaglia and Y. Tian, Chem. Commun., 2017, 53, 3303 - 3306. c) L. K. McKenzie, I. V. Sazanovich, E. Baggaley, M.

Bonneau, V. Guerchais, J. A. G. Williams, J. A. Weinstein and H. E. Bryant, Chem. Eur. J.,

SC R

2017, 23, 234 - 238. d) F. Xue, Y. Lu, Z. Zhou, M. Shi, Y. Yan, H. Yang and S. Yang,

Organometallics, 2015, 34, 73 - 77. e) S. Mandal, D. K. Poria, R. Ghosh, P.S. Ray and P. Gupta, Dalton Trans., 2014, 43, 17463 - 17474.

U

[8] M. Montalti, A. Credi, L. Prodi and M. T. Gandolfi, Handbook of Photochemistry, 3rd

N

ed.; CRC Press, 2006, pp 621.

Risler,

N.

Langer,

G.

Wagenblast,

I.

Münster

and

T.

Strassner,

M

A.

A

[9] a) G. R. Kumar and P. Thilagar, Inorg. Chem., 2016, 55, 12220 - 12229. b) A. Tronnier,

Organometallics, 2012, 31, 7447 - 7452. c) J. Sanning, L. Stegemann, P. R. Ewen, C.

ED

Schwermann, C. G. Daniliuc, D. Zhang, N. Lin, L. Duan, D. Wegner, N. L. Doltsinis and C. A. Strassert, J. Mater. Chem. C, 2016, 4, 2560 - 2565. d) F. Julia, D. Bautista and P. Gonza´lez-Herrero, Chem. Commun., 2016, 52, 1657 - 1660. e) A. K. Chan, M. Ng, Y.

PT

Wong, M. Chan, W. Wong and V. W. Yam, J. Am. Chem. Soc., 2017, 139, 10750 - 10761. f) T. Kayano, S. Takayasu, K. Sato, and K. Shinozaki, Chem. Eur. J., 2014, 20, 16583 – 16589.

CC E

g) V. V. Sivchik, E. V. Grachova, A. S. Melnikov, S. N. Smirnov, A. Yu. Ivanov, P. Hirva, S. P. Tunik and I. O. Koshevoy, Inorg. Chem., 2016, 55, 3351 - 3363. h) K. M. Wong and V. W. Yam Acc. Chem. Res., 2011, 44, 424 - 434. i) T. Abe, T. Itakura, N. Ikeda and K.

A

Shinozaki, Dalton Trans., 2009, 711 - 715. [10] a) J. R. Berenguer, E. Lalinde, A. Martin, M. T. Moreno, S. Sanchez and H. R. Shahsavari, Inorg. Chem., 2016, 55, 7866 - 7878. b) S. Fernandez, J. Fornies, B. Gil, J. Gomez and E. Lalinde, Dalton Trans., 2003, 5, 822 - 830. c) M. -C. Tse, K. -K. Cheung, M. C. -W. Chan and C. -M. Che, Chem. Commun., 1998, 21, 2295 - 2296.

12

[11] a) H. -Q. Liu, T. -C. Cheung and C. -M. Che, Chem. Commun., 1996, 9, 1039 - 1040. b) P. I. Kvam, M.V. Puzyk, K. P. Balashev and J. Songstad, Acta Chem. Scand., 1995, 49, 335 343. c) P. I.; Kvam and J. Songstad, Acta Chem. Scand., 1995, 49, 313 - 324. d) K. P. Balashev, M. A. Ivanov, T. V. Taraskina and E. A. Cherezova, Russ. J. Gen. Chem., 2006, 76, 781 - 790. e) D. -L. Ma, C. -M. Che and S. -C. Yan, J. Am. Chem. Soc., 2009, 131, 1835 - 1846. f) A. Ionescu and L. Ricciardi, Inorg. Chim. Acta, 2017, 460, 165 170. g) K. P. Balashev, M. V. Puzyk, V.S. Kotlyar and M. V. Kulikova, Coord. Chem.

IP T

Rev., 1997, 159, 109 - 120.

[12] a) G. Li, A. Wolfe, J. Brooks, Z. Q. Zhu and J. Li, Inorg. Chem., 2017, 56, 8244 -

SC R

8256. b) T. R. Schulte, J. J. Holstein, L. Krause, R. Michel, D. Stalke, E. Sakuda, K.

Umakoshi, G. Longhi, S. Abbate and G. H. Clever, J. Am. Chem. Soc., 2017, 139, 6863 6866. c) A. F. Henwood and E. Zysman-Colman, Chem. Commun., 2017, 53, 807 - 826. d)

Faghih,

Z.

Faghih,

Z.

Ahmadipour,

B.

U

M. Fereidoonnezhad, B. Kaboudin, T. Mirzaee, R. B. Aghakhanpour, M. G. Haghighi, Z. Notash

and

H.

R.

Shahsavari,

N

Organometallics, 2017, 36, 1707 - 1717. e) M.A.; Esteruelas, E. Oñate and A. U. Palacios,

A

Organometallics, 2017, 36, 1743 - 1755. f) M.Z. Shafikov, D.N. Kozhevnikov, M.

M

Bodensteiner, F. Brandl and R. Czerwieniec, Inorg. Chem., 2016, 55, 7457 - 7466. [13] a) A. Patel, S. Y. Sharp, K. Hall, W. Lewis, M. F. G. Stevens, P. Workman and C. J.

ED

Moody, Org. Biomol. Chem., 2016, 14, 3889 - 3905. b) J. Z. Wu, L. Wang, G. Yang and L. N. Ji, Chin. Chem. Lett., 1995, 11, 999 - 1002. c) J. Z. Wu, G. Yang, S. Chen, L. N. Ji, J. Y.

PT

Zhou and Y. Xu, Inorg. Chim. Acta, 1998, 283, 17 - 23.

CC E

[14] a) B. U. Coban, U. Yildiz and A. Sengul, J. Biol. Inorg. Chem., 2013, 18, 461 - 471. [15] S. W. Thomas III, K. Venkatesan, P. T. Mueller and M. Swager, J. Am. Chem. Soc., 2006, 128, 16641 - 16648.

A

[16] a) J. A. Bailey, M. G. Hill, R. E. Marsh, V. M. Miskowski, W. P. Schaefer and H. B. Gray, Inorg. Chem., 1995, 34, 4591 - 4599. b) E. Kabir, C. H. Wu, J. I. Wu and T. S. Teets, Inorg. Chem., 2016, 55, 956 - 963. [17] a) J. A. G. Williams, A. Beeby, E. S. Davies, J. A. Weinstein and C. Wilson, Inorg. Chem., 2003, 42, 8609 - 8611. b) S. J. Farley, D. L. Rochester, A. L. Thompson, J. A. K. Howard and J. A. G. Williams, Inorg. Chem., 2005, 44, 9690 - 9703. c) B. Ma, P. I. 13

Djurovich and M. E. Thompson, Coord. Chem. Rev., 2005, 249, 1501 - 1510. d) G. A. Crosby and K. R. Kendrick, Coord. Chem. Rev., 1998, 171, 407 - 417. e) C. N. Pettijohn, E. B. Jochnowitz, B. Chuong, J. K. Nagle and A. Vogler, Coord. Chem. Rev., 1998, 171, 85 92. f) H. Kunkely and A. Vogler, J. Am. Chem. Soc., 1990, 112, 5625 - 5627. [18] a) H. Imoto, S. Tanaka, T. Kato, S. Watase, K. Matsukawa, T. Yumura and K. Naka, Organometallics, 2016, 35, 364 - 369. b) S. Culham, P. H. Lanoë, V. L. Whittle, M. C.

IP T

Durrant, J. A. G. Williams and V.N. Kozhevnikov, Inorg. Chem., 2013, 52, 10992 - 11003.

[19] P. Pinter, H. Mangold, I. Stengel, I. Münster and T. Strassner, Organometallics, 2016,

SC R

35, 673 - 680. [20] K. Nakamaru, Bull. Chem. Soc. Jpn., 1982, 55, 1639 - 1640.

U

[21] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P.

N

Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K.

A

Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T.

M

Vreven, J. A., Jr. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,

ED

J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morukuma, V. G.

PT

Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Ciolowski and D. J. Fox, Gaussian 09. B.01; Gaussian,

CC E

Inc., Wallingford, CT, 2009.

[22] H. Leopold, M. Tenne, A. Tronnier, S. Metz, I. Münster, G. Wagenblast and T.

A

Strassner, Angew. Chem. Int. Ed. 2016, 55, 15779 - 15782.

Figures Figure 1: Geometry optimized figure of complexes 1 - 3

14

IP T

Figure 2: Electronic absorption spectra of complexes 1-3 a) in DMF, conc. 10-5M, b) in DMF conc. 10-3M, c) in solid state. d) Excitation spectra against em = 545(1), 540(2), 547(3) b

SC R

a

4

2

Solvent: DMF Conc.: 1mM

Solvent: DMF -5

Conc.: 10 mM

600 Wavelength(nm)

700

2

A

N

1

U

500

Absorption

Absorbance

Absorbance

0

1 2 3

1 2 3

2

0

0 300

400

500

600

700

800

500

600

700

Wavelength(nm)

M

Wavelength(nm)

c

d

2.0

ED

0

600

CC E

400

800

Excitation spectra em ~ 540 nm

1.8

Excitation intensity

1

PT

Absorbance

1 2 3

1.6

1 2 3

1.4 1.2 1.0 0.8 0.6 0.4 0.2

1000

0.0 300

Wavelength (nm)

350

400

450

Wavelength(nm)

A

Figure 3: Emission spectra of complexes 1-3 in a) degassed DMF b) solid state

15

500

a

b 7

7

Emission intensity

2.5x10

ex = 500 nm

7

ex = 520nm

1.2x10

1 2 3

Solvent: DMF -3 Conc.: 10 M

1 2 3

7

1.0x10

Emission Intensity

3.0x10

7

2.0x10

7

1.5x10

7

1.0x10

6

8.0x10

6

6.0x10

6

4.0x10

6

2.0x10

6

5.0x10

0.0 600

0.0 700

650

700

750

800

Wavelength (nm)

750

IP T

600 650 Wavelength (nm)

N

U

SC R

550

A

Figure 4: Photographic images of the emission color for complex 1-3 in solid state in an open

CC E

PT

ED

M

atmosphere. The excitation wavelength is 366 nm

A

Figure 5: Geometry optimized figure of dimeric complex 2

16

IP T

SC R

Figure 6: Emission spectra of complex 1 with 450 nm commercial LED (as excitation source)

-5.0

U

-5.5 -6.0

-7.5 -8.0 -8.5 -9.0

3.5

4.0

2

3

PT

1

3.0

ED

2.5

3 . 0 8 3

M

-7.0

A

2.286

E/eV 3.002 2.0

N

-6.5

CC E

Figure 7: Energy level, energy gaps (in eV), and selected Selected frontier molecular orbitals

A

involved in the excitation and the singlet excited state

17

IP T SC R

Figure 8: The excited states ordering of complexes 1 - 3

U

Scheme 1: Details of synthesis of Ligand L1 - L3 and complexes 1 - 3,

N

with procedures starting from 1,10-phenanthroline as ligand

O

A

and K2PtCl4 as metal precursor O

H N

N

NH4OAc, Reflux, 2h

a

b

Ar =

L1 (a, c, e = H, b,d = CF3) L2 (a - e = H) L3 (a,b,d, e = H, c = NMe2)

PT

c e

d

CC E

2-phenylpyridine

K2PtCl 4

Cl

N

Pt

Pt Cl

A

2-methoxyethanol-H2O (3:1), Reflux, 100oC

N

Pt Cl

N

N

Cl L1 - L3

Cl Pt

Ar

N

N

ED

N

M

Ar-CHO N Glacial Acetic acid

DMF, 70oC 24h

N

N

H N

N

N

Ar

Pt

Scheme 1

18

Table 1: Photophysical parameters for the complexes 1 - 3 λema,d,e

λemc,e (nm)

(nm) (b)

(nm)

(nm)

487(0.40,

536, 455, 545(10-6M), 545, 631, 713

415(0.67),

404, 380

663 (10-4M)

306(8.50)

666 (10-3M)

5.46

14.5

506(0.35),

565,

540(10-6-10-5M),

413(1.51),

440,

540, 657(10-4M)

353(9.03),

381

657 (10-3M)

538,

547(10-6-10-5M),

15.02

3.46,

697

15. 47

SC R

354(7.40),

3.6,

(%)

IP T

628(10-5M)

1.08

12.46

19.31

664 (10-3M)

A

535(0.67),

3.63,

649

5.2 7

-

-

ED

317(18.21)

4.38

4.86

M

351(16.04) 393

a

c (s) c,h

(%)

335(9.64) 3

a (s) f

U

2

λabsc

N

1

λabsa

DMF. b10-4M−1cm−1. csolid. dλex = 466 nm. eλex = 520 nm. fThe quantum yield in

degassed dimethylformamide solution was estimated relative to [Ru(bpy)3](PF6)2 in

CC E

PT

acetonitrile as the standard (em = 5.9%). hAbsolute quantum yield.

Table

2:

Electrochemical

Data

Measured

at

Room

Temperature

in

degassed

dimethylformamide and calculation of excited state potential of the complexes 1, 2 and 3

A

Complex Eox

1

(V

vs Ered (V vs Eox* (V vs Ag/AgCl)

Ag/AgCl)

Ag/AgCl)

1.14

-0.38, -0.95, -0.81

Ered* (V vs Ag/AgCl)

1.57

-1.23, -1.50 2

0.99

-0.39, -0.57, -0.87 19

1.47

-0.83, -1.27 -0.36, -0.87, -0.94

0.97, 1.15

3

1.55

-1.32

IP T

Table 3: Correlation of electronic spectral data with the calculated (DFT-TDDFT) spectral

1

(nm,

force

Composition(

constant)(b)

%)

519(0.0411)(487)

HL(96)

416(0.1204)( 415)

ema

Composition

Character

(b)

(%)

(ppy-*(L1),

540

H L+6

LLCT,

MLCT

(545)

H L+7

MLCT

571(540)

H L+6

LLCT,

Character

H-1L(81)

MLCT

H-2

A

347(0.1638)( 354)

L+1(78) H  L+5

506(0.0382)(506)

HL (93)

CC E

382(0.1770)(353)

A

306(0.3486)(335)

3

554(0.1327)(535)

( ppy)-*(ppy) (ppy)-*(L2), MLCT H L+7  (ppy)-*(L2)

H-1 L+1 (80)

( ppy/L2)-*(L2)

H-1L+3 (48)

(ppy/L2)*(ppy/L2)

HL+1

(L3)-*(L3),

(98)

LMCT

400(0.2334)(351)

394(0.5881)(317)

MLCT

H-2 (74)

PT

399(0.0797)(413)

ED

(55)

M

(ppy/L1-*(L1)

312(0.0980)( 306)

2

N

(ppy-*(L1),

U

Absa

SC R

assignment (H = HOMO and L = LUMO).

559(547)

H L+4

LLCT,

H L+5

MLCT

H L+6 H-2  L

(ppy/L3)-

(58)

*(ppy/L3), MLCT

H  L+3

(L3)-*(ppy/L3)

20

H L+7

304(0.1694)

(72)  (L3)-*( ppy/L3)

H -7 L (24)

calculated b experimental

A

CC E

PT

ED

M

A

N

U

SC R

IP T

a

21