Influence of Ancillary Ligands and Isomerism on the Luminescence of

Article pubs.acs.org/IC

Influence of Ancillary Ligands and Isomerism on the Luminescence of Bis-cyclometalated Platinum(IV) Complexes Fabio Juliá,† María-Dulce García-Legaz,† Delia Bautista,‡ and Pablo González-Herrero*,† †

Departamento de Química Inorgánica, Facultad de Química and ‡SAI, Universidad de Murcia, Apdo. 4021, 30071 Murcia, Spain S Supporting Information *

ABSTRACT: The synthesis, characterization, and photophysical properties of a wide variety of bis-cyclometalated Pt(IV) complexes featuring a C2-symmetrical or unsymmetrical {Pt(ppy)2} unit (sym or unsym complexes, respectively; ppy = Cdeprotonated 2-phenylpyridine) and different ancillary ligands are reported. Complexes sym-[Pt(ppy)2X2] (X = OTf−, OAc−) were obtained by chloride abstraction from sym-[Pt(ppy)2Cl2] using the corresponding AgX salts, and the triflate derivative was employed to obtain homologous complexes with X = F−, Br−, I−, trifluoroacetate (TFA−). Complexes unsym-[Pt(ppy)2(Me)X] (X = OTf−, F−) were prepared by reacting unsym[Pt(ppy)2(Me)Cl] with AgOTf or AgF, respectively, and the triflate derivative was employed as precursor for the synthesis of the homologues with X = Br−, I−, or TFA− through its reaction with the appropriate anionic ligands. The previously reported complexes unsym-[Pt(ppy)2X2] (X = Cl−, Br−, OAc−, TFA−) are included in the photophysical study to assess the influence of the arrangement of the cyclometalated ligands. Density functional theory (DFT) and time-dependent DFT calculations on selected derivatives were performed for a better interpretation of the observed excited-state properties. Complexes sym[Pt(ppy)2X2] (except X = I−) exhibit phosphorescent emissions in fluid solutions at 298 K arising from essentially 3LC(ppy) excited states, which are very similar in shape and energy. However, their efficiencies are heavily dependent on the nature of the ancillary ligands, which affect the energy of deactivating ligand-to-ligand charge transfer (LLCT) or ligand-to-metal charge transfer (LMCT) states. The fluoride derivative sym-[Pt(ppy)2F2] shows the highest quantum yield of this series (Φ = 0.398), mainly because the relatively high metal-to-ligand charge transfer admixture in its emitting state leads to a high radiative rate constant. Complexes unsym-[Pt(ppy)2X2] emit from 3LC(ppy) states in frozen matrices at 77 K, but their emissions are totally quenched in fluid solution at 298 K because of the presence of low-lying, dissociative LMCT excited states, which also cause photoisomerization reactions. Complexes unsym-[Pt(ppy)2(Me)X] (X = F−, Cl−, Br−, TFA−) show strong emissions in fluid solutions at 298 K (Φ = 0.52−0.63) because deactivating LMCT states lie at high energies. However, derivative unsym[Pt(ppy)2(Me)I] is only weakly emissive at 298 K because of the presence of low-lying LLCT [p(I) → π*(ppy)] states.

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INTRODUCTION The study of the photophysical and photochemical properties of transition metal complexes with heterocyclic aromatic ligands is essential to research areas concerned with the interconversion between light and chemical or electrical energy or the exploitation of photo- or chemiluminescence in chemical analysis and bioimaging.1 These include the development of photosensitizers for dye-sensitized solar cells,2 electroluminescent materials for lighting and display applications,3 photoredox catalysts for water splitting4 or organic synthesis,5 chemosensors,6 and luminescent probes for cellular imaging.7 The modulation or improvement of excited-state properties has been exhaustively sought to better suit specific applications, either through ligand derivatization or by using different metal ions. In particular, the quest for stable complexes that exhibit highly efficient phosphorescent emissions, relatively long excited-state lifetimes, and tunable emission colors has been intensive and remains an important research objective. © 2016 American Chemical Society

Although polypyridine complexes of Ru(II) and Os(II) have dominated the historical development of this field,8 Ir(III) and Pt(II) complexes with cyclometalated arylpyridines and related ligands have become the most studied systems in recent years because they display highly tunable emission energies and can reach very high quantum yields.9 These complexes usually exhibit emitting triplet excited states of mixed ligand-centered/ metal-to-ligand charge-transfer character (3LC/MLCT). The proportion of the MLCT contribution largely determines the excited-state properties, because it correlates with the effectiveness of the spin−orbit coupling induced by the metal, which promotes intersystem crossing to the triplet excited state and also accelerates the radiative transition to the ground state.10 Received: May 4, 2016 Published: July 20, 2016 7647

DOI: 10.1021/acs.inorgchem.6b01100 Inorg. Chem. 2016, 55, 7647−7660

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Inorganic Chemistry In contrast to the exhaustive studies on other d6 ions, the excited-state properties of Pt(IV) complexes have been only scarcely studied, probably because of the intense attention devoted to Pt(II) systems. The first Pt(IV) organometallic emitters were bis-cyclometalated neutral complexes of the type [Pt(C^N)2(R)Cl] [C^N = cyclometalated 2-phenylpyridine (ppy) or 2-thienylpyridine (thpy); R = CH2Cl, CHCl2], which exhibited phosphorescence in solution at room temperature with quantum yields in the range of 0.05−0.15.11 More recently, dicationic complexes of the type [Pt(C^N)2(N^N)]2+ (N^N = aromatic diimine) were reported to exhibit quantum yields in the range of 0.0005−0.035.12 In addition, fluorescent Pt(IV) complexes have been obtained by attaching highly conjugated chromophoric ligands;13 compounds of this class are of interest in the field of cancer chemotherapy, because their localization and activity within tumor cells can be monitored by fluorescence imaging.14 We recently started the development of highly efficient, phosphorescent Pt(IV) emitters and reported a series of meridional (mer) and facial ( fac) isomers of cationic homoleptic tris-cyclometalated complexes mer/fac-[Pt(C^N) 3]+, as well as heteroleptic derivatives mer-[Pt+ ̂ , which exhibit quantum yields (Φ) up to (C^N)2(C′^N′)] 15 0.49. The emission efficiencies of these complexes are strongly dependent on the type of isomer, the fac derivatives being considerably more efficient than their mer counterparts. We also reported a series of neutral bis-cyclometalated chloro(methyl)platinum(IV) complexes, characterized by a high photostability, tunable emission energies, and quantum efficiencies that can reach the highest values ever found for Pt(IV) emitters (up to 0.81).16 Cyclometalated Pt(IV) complexes emit from essentially 3LC states with very little MLCT character mixed in. The diminished MLCT contribution with respect to cyclometalated Ir(III) or Pt(II) complexes is a consequence of the very low energy of the occupied metal d orbitals in Pt(IV), although it is still sufficient to facilitate the formation of the triplet emitting state and preclude fluorescence in most cases. Concomitantly, ligand-to-metal charge-transfer (LMCT) states may become thermally accessible from the emitting state, thus providing nonradiative deactivation pathways in some Pt(IV) derivatives.15b In the light of our previous results, we believe that cyclometalated Pt(IV) complexes represent a valuable class of luminescent compounds and that their photophysical and photochemical properties need to be studied more in depth to provide a useful basis for further developments. Herein, we present a fundamental photophysical study on systematically derivatized bis-cyclometalated Pt(IV) complexes containing 2phenylpyridine (ppy) as the cyclometalating ligand, aimed at the elucidation of the effects of ancillary ligands and isomerism on their luminescence. The studied complexes may feature a C2-symmetrical (Chart 1, type A) or unsymmetrical (types B and C) {Pt(ppy)2} unit and a variety of monodentate ligands in the two remaining, cis-disposed coordination positions. For simplicity, the tags sym and unsym will be employed throughout this article to denote the arrangement of the ppy ligands and do not necessarily refer to the overall symmetry or stereochemistry of the molecule. Many of the studied complexes exhibit phosphorescent emissions at room temperature in degassed fluid solutions, arising from essentially 3LC states based on the ppy ligands. Consistent with the character of the emitting state, the emission energies do not show significant variations along the series. However, we found that the nature of the ancillary

Chart 1

ligands and the arrangement of the cyclometalating ligands have a significant impact on the quantum yields because they may modulate the extent of metal-orbital involvement in the excited state or determine the existence and/or energy of thermally accessible deactivating excited states.

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RESULTS AND DISCUSSION Synthesis. The synthesis of the studied bis-cyclometalated Pt(IV) complexes was achieved by using Pt(II) precursors that predetermine the mutually trans or cis arrangement of the pyridyl moieties (Schemes 1 and 2). Thus, for the preparation

Scheme 1a

(a) PhICl2, − HCl. (b) 2 AgOAc, − 2 AgCl. (c) 2 AgOTf, − 2 AgCl. (d) 2 Bu4NF, − 2 Bu4NOTf or 2 NaX, − 2 NaOTf. a

of complexes featuring a C2-symmetrical {Pt(ppy)2} unit, we employed [Pt(ppy)(ppyH)Cl]17 as the primary precursor, in which the two pyridyl moieties are mutually trans; the oxidation of this complex with PhICl2 leads to the metalation of the pendant phenyl group of the ppyH ligand to give sym[Pt(ppy)2Cl2] (sym-Cl) in high yield.12,18 The substitution of the chlorides in sym-Cl by other ligands has been previously achieved by using AgOTf.12,15 For the present work, we performed the reactions of sym-Cl with 2 equiv of AgOTf in refluxing acetone or with AgOAc in 1,2-dichloroethane at 90 °C in the absence of added ligands, which allowed the isolation of sym-[Pt(ppy)2(OTf)2] (sym-OTf) or sym-[Pt(ppy)2(OAc)2] (sym-OAc), respectively, containing monodentate triflate or acetate ligands. Complex sym-OTf is of extraordinary synthetic utility, because the lability of the weakly coordinating triflate allows a very easy substitution by other ligands. In contrast to sym-Cl or sym-OAc, complex sym-OTf is rather soluble in coordinating solvents, such as acetone, MeCN, or tetrahydrofuran (THF), presumably because they displace the OTf− ligands. Therefore, the rest of derivatives of this class, sym[Pt(ppy)2(X2)] [X = F− (sym-F), Br− (sym-Br), I− (sym-I), 7648


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Inorganic Chemistry Scheme 2a

Pt(II) precursors with XeF2 or through halogen exchange in Pt(IV) complexes using AgF.19 The unsymmetrical complexes unsym-[Pt(ppy)2X2] (unsymX; X = Cl−, Br−, OAc−, TFA−) were prepared by reacting cis[Pt(ppy)2]20 with PhIX2 (X = Cl−, OAc−, TFA−) or Br2, following the procedure described by Whitfield and Sanford.21 The oxidation of methyl Pt(II) complexes of the type [Pt(C^N)(N^CH)(Me)Cl] (Scheme 2), in which the pyridyl moieties are in mutually cis positions, was recently reported by us16 and provides another route to bis-cyclometalated Pt(IV) derivatives featuring an unsymmetrical {Pt(ppy)2} unit. For the present study, the complex [Pt(ppy)2(Me)Cl] (unsym-MeCl)16 was employed as a precursor for further derivatization. The substitution of the chloride ligand in this complex by fluoride or triflate was achieved by using the corresponding silver salts to give [Pt(ppy)2(Me)X] [X = F− (unsym-MeF) or OTf− (unsymMeOTf)]. The triflate complex was then employed for the preparation of the rest of methyl complexes [Pt(ppy)2(Me)X] [X = Br− (unsym-MeBr), I− (unsym-MeI), TFA− (unsymMeTFA)] through its reaction with the appropriate sodium salts. Crystal Structures. The crystal structures of sym-OTf, symBr·CH2Cl2, sym-TFA, unsym-MeF, and unsym-MeBr were solved by single-crystal X-ray diffraction studies. Thermal ellipsoid plots are shown in Figure 1, and selected bond distances and angles are listed in Table 1. The molecules of sym-OTf lie along a crystallographic twofold axis, and therefore the asymmetric unit in this structure contains only half of the molecule. All complexes exhibit the expected octahedral coordination environment around the Pt atom, which is slightly distorted because of the narrow bite angle of the ppy

a (a) PhIX2 or Br2. (b) PhICl2, (i-Pr)2EtN, − (i-Pr)2EtNHCl, − PhI. (c) AgF, − AgCl. (d) AgOTf, − AgCl. (e) NaX, − NaOTf.

TFA− (sym-TFA; TFA = trifluoroacetate)], were obtained by reacting sym-OTf with the appropriate anionic ligands under mild conditions. The successful synthesis of sym-F using this methodology is remarkable, since organometallic Pt(IV) difluorides are uncommon and have been obtained by reacting

Figure 1. Thermal ellipsoid representations of the crystal structures of sym-OTf (a), sym-TFA (b), sym-Br·CH2Cl2 (c), unsym-MeF (d), and unsymMeBr (e) (50% probability, except sym-TFA, 30%). The solvent molecule in sym-Br·CH2Cl2 and H atoms in all cases are omitted. 7649


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Inorganic Chemistry Table 1. Selected Bond Distances (Å) and Angles (deg) for the Crystal Structures Reported in This Work sym-OTf

sym-TFA

sym-Br

unsym-MeF

unsym-MeBr

Pt−C1 Pt−O1 C1−Pt−N1 O1−Pt−O1#1 Pt−C1 Pt−C12 Pt−N1 C1−Pt−N1 O3−Pt−O1 Pt−C1 Pt−C12 Pt−N1 C1−Pt−N1 Br1−Pt−Br2 Pt−C1 Pt−C12 Pt−C23 C1−Pt−N1 C23−Pt−F1 Pt−C12 Pt−C1 Pt−C23 C1−Pt−N1 C23−Pt−Br1

1.9930(17) 2.1947(12) 82.00(7) 81.26(7) 2.013(7) 2.018(7) 2.028(5) 83.1(3) 87.2(2) 2.017(4) 2.012(5) 2.033(4) 81.61(17) 92.338(15) 2.003(3) 1.988(3) 2.042(3) 80.82(12) 89.10(11) 2.020(2) 2.022(3) 2.063(5) 80.30(9) 88.87(19)

(N−Pt−C: 80.01−83.90°) ligand. The sym-X complexes show Pt−N distances in the range of 2.025−2.033 Å corresponding to the mutually trans pyridyl moieties, which are similar to those found for sym-[Pt(C^N)2Cl2] [C^N = ppy (sym-Cl) or 2(4-fluorophenyl)pyridine]22 and complexes of the type mer+ 15 ̂ . In the unsym-MeX derivatives, the [Pt(C^N)2(C′^N′)] pyridyl moieties are trans to an aryl moiety or a methyl ligand, and the corresponding Pt−N distances are longer (2.125− 2.139 Å) because of the strong trans influences of these groups. Similarly, the Pt−C bond distances correlate with the trans influence of the ancillary ligands in all cases. This can be clearly observed by examining the structures of the series sym-X, in which the Pt−C distances increase from 1.993 to 2.018 Å along the sequence X = OTf− < Cl− [2.007(6), 2.030(5) Å]22 < TFA− ≈ Br−, that is, as the trans influence of the X ligands increases. Analogously, in derivatives unsym-MeX, the Pt−C distances trans to X increase in the sequence X = F− [1.988(3) Å] < Cl− [2.009(5) Å]16 < Br− [2.020(2) Å]. The case of unsym-MeF is especially remarkable, since it shows the shortest Pt−C bond distance within the studied complexes because of the very low trans influence of the fluoride ligand. There are only a few crystal structures of Pt(IV) complexes with a fluoride ligand trans to an aryl in the Cambridge Structural Database, the majority of which exhibit very short Pt−C(aryl) bond distances (range of 1.973−2.085 Å).19,23 Electronic Absorption Spectra. The UV−vis absorption spectra of the studied complexes were registered in CH2Cl2 solutions at 298 K. The resultant data are summarized in Table 2, and the spectra are shown in Figures 2 and 3. The triflate complexes were excluded from the photophysical study because their low stability in solution precluded the obtention of reliable data. The derivatives featuring a C2-symmetrical {Pt(ppy)2} unit give rise to intense, structured bands in the range of 300− 350 nm with a characteristic peak around 310 nm and the lowest-energy maximum around 335 nm, which are similar to those found for fac-[Pt(ppy)3]OTf15a and can be ascribed to

Pt−N1

2.0250(15)

N1−Pt−O1

91.55(5)

Pt−N2 Pt−O1 Pt−O3 C12−Pt−N2

2.032(6) 2.159(6) 2.136(5) 83.9(3)

Pt−N2 Pt−Br1 Pt−Br2 C12−Pt−N2

2.028(4) 2.5785(5) 2.5901(5) 81.31(18)

Pt−F1 Pt−N1 Pt−N2 C12−Pt−N2

2.0889(19) 2.136(3) 2.125(3) 80.88(12)

Pt−N2 Pt−N1 Pt−Br1 C12−Pt−N2

2.127(2) 2.139(2) 2.5674(4) 80.01(9)

Table 2. Electronic Absorption Data for the Studied Complexes in CH2Cl2 Solution (ca. 5 × 10−5 M) at 298 K complex sym-F sym-Cl sym-Br sym-I sym-OAc sym-TFA unsym-Cl unsym-Br unsym-OAc unsym-TFA unsym-MeF unsym-MeCl unsym-MeBr unsym-MeI unsymMeTFA

λmax, nm (ε × 10−3, M−1 cm−1) 262 (12.9), 270 (12.4), 309 (9.1), 327 (7.0), 338 (7.1) 259 (20.9), 269 (18.7), 308 (11.9), 324 (10.7), 335 (10.7) 261 (23.0), 269 (21.4), 309 (12.0), 326 (9.1), 338 (9.3) 260 (33.0), 310 (17.3), 339 (10.7), 378 (1.1, sh) 260 (21.8), 308 (13.5), 322 (11.6), 334 (11.6) 259 (20.6), 268 (17.4), 310 (13.7), 321 (13.0), 332 (12.2) 266 (25.4), 310 (16.8), 324 (13.4, sh), 342 (5.7, sh) 260 (23.2), 267 (22.7), 307 (15.4), 325 (11300, sh), 342 (5.20, sh) 260 (19.8), 309 (13.6), 323 (10.6, sh), 339 (6.0, sh) 265 (18.6), 309 (14.2), 323 (11.0, sh), 341 (5.1, sh) 263 (24.5), 270 (23.4, sh), 306 (14.0), 323 (11.4), 333 (10.6, sh) 263 (25.0), 270 (24.0, sh), 306 (14.9), 321 (12.7), 332 (11.8, sh) 263 (23.5), 270 (23.5, sh), 306 (13.8), 323 (11.4), 333 (10.8, sh) 261 (21.8), 270 (20.4, sh), 306 (12.7), 325 (8.7), 336 (7.7, sh), 378 (0.2, sh) 262 (22.3), 269 (20.7, sh), 307 (14.3), 321 (12.5), 333 (11.0, sh)

predominantly 1LC transitions within the ppy ligands (π−π*). In agreement with this assignment, these absorptions remain mostly unaffected by the variation of the ancillary ligands. In addition, low-intensity bands clearly overlap the slope of the lowest-energy maximum in the case of sym-I, resulting in a long tail extending to ca. 415 nm. Given that the iodide ligand possesses a significant π-donating ability due to the presence of available (high-lying) lone pairs, the possible origin of these bands is ligand-to-ligand charge-transfer [LLCT; I−→π*(ppy)] and/or LMCT [I−→dσ*(Pt)] transitions. A close examination of the lowest-energy slope of the rest of derivatives of this class showed that it shifts to lower energies along the sequence 7650


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Inorganic Chemistry

common to both series of complexes and must arise predominantly from 1LC(ppy) transitions. However, the lowest-energy band is somewhat red-shifted and much less intense for unsym-X derivatives, occurring at ca. 342 nm with tails up to 400 nm. The whole spectra are very similar to that of mer-[Pt(ppy)3]OTf,15 whose lowest-energy band comprises mainly 1LC(ppy) transitions with some 1LLCT [π(ppy) → π*(ppy′)] character, due to the presence of inequivalent ppy ligands.15 An analogous assignment of the lowest absorption is reasonable for unsym-X derivatives, which would also be consistent with the fact that this band does not show any dependence on the nature of the ancillary ligands. The absorption spectra of complexes unsym-MeX are in general very similar to those found for the sym-X series, although their maxima are slightly blue-shifted and do not show significant changes with the variation of ligand X−, which indicates that they arise mostly from 1LC(ppy) transitions. The slope of the lowest-energy maximum (Figure S5) shifts to lower energies in the sequence TFA− < Cl− ≈ Br− < F− < I−, which correlates with the π-donating ability of the X− ligands, again with the exception of fluoride. Luminescence. The excitation and emission spectra of the studied complexes were measured in deaerated CH2Cl2 solutions at 298 K and in frozen butyronitrile (PrCN) or 2methyltetrahydrofuran (MeTHF) glasses at 77 K. Emission data are summarized in Table 3, and emission spectra at 298 K are shown in Figure 4. With the exception of sym-I, the complexes with a C2symmetrical {Pt(ppy)2} unit are luminescent in deaerated fluid solutions at 298 K, giving rise to almost identical emissions (Figure 4), with a noticeable vibrational structure, large Stokes shifts, and lifetimes in the microsecond range. All complexes of this series are emissive in frozen matrices at 77 K, showing sharply structured bands that, in general, are only slightly shifted to higher energies with respect to the room-temperature emissions, probably because of a small rigidochromic effect.24 These characteristics indicate that in all cases the emissions arise from a triplet excited state essentially centered on the ppy ligands (3LC), which must also involve a small degree of MLCT admixture, as already noted for previously reported cyclometalated Pt(IV) complexes.15,16 Interestingly, although the ancillary ligands do not have an observable effect on the shape or energy of the emission band, they do have a critical influence on the emission intensities at room temperature. Thus, the fluorocomplex sym-F exhibits a remarkable quantum yield of 0.398, the highest within this series, while the rest of the ancillary ligands lead to significantly diminished emission efficiencies. An examination of radiative and nonradiative rate constants (kr and knr) revealed that the differences in the emissive behavior of these complexes are mostly dictated by nonradiative processes. In effect, knr values increase with the πdonating ability of the ancillary ligands. This is clearly observed for the halide series: while sym-F and sym-Cl display similar knr values, the weakly emissive sym-Br shows a dramatically increased knr, and that of the nonemissive sym-I derivative is expected to be even higher. The same trend is observed when comparing sym-TFA with sym-OAc. This suggests that thermally accessible LLCT [p(X) → π*(ppy)] or LMCT [p(X) → dσ*(Pt)] states may facilitate nonradiative deactivation in compounds sym-X. Notwithstanding, the very different quantum yields displayed by sym-F and sym-Cl cannot be explained solely on the basis of their knr values, which differ only slightly. Rather, the much higher efficiency of sym-F must

Figure 2. Electronic absorption spectra of complexes sym-X in CH2Cl2 solution at 298 K (ca. 5 × 10−5 M).

Figure 3. Electronic absorption spectra of unsym-X (top) and unsymMeX (bottom) derivatives in CH2Cl2 solution at 298 K (ca. 5 × 10−5 M).

TFA− < Cl− < F− ≈ Br− < AcO− (Figures 2 and S5), which appears to correlate with the π-donating ability of the ancillary ligands (except for fluoride; see the Computational study), suggesting that similar LLCT or LMCT states may be present in some of these cases. Complexes unsym-X show almost identical, structured absorption profiles (Figure 3). A comparison with those of sym-X isomers showed that the bands at ∼310 and 323 nm are 7651


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Inorganic Chemistry Table 3. Emission Data of the Studied Complexes 298 Ka Φ

−3

sym-F sym-Cl sym-Br sym-I sym-OAc sym-TFA unsym-Cl unsym-Br unsym-OAc unsym-TFA unsym-MeF unsym-MeCl unsym-MeBr unsym-MeI unsym-MeTFA

451 450 451

0.398 0.114 0.0034

24.1 27.0 0.46

16.5 4.2 7.4

25.0 32.8 2167

452 452

0.027 0.038

6.5 10.5

4.1 3.6

149.7 91.6

447 447 447 447 447

0.515 0.602 0.633 0.0056 0.555

73.5 99.7 119 0.33 102

7.01 6.04 5.32 17.0 5.44

6.60 3.99 3.08 3013 4.36

e

kr × 10

−1 f

λem (nm)

d

τ (μs)

77 Kb −3

complex

c

(s )

knr × 10

−1 g

(s )

λem (nm) 448 445 447 446 446 447 446 446 446 447 447 442 442 443 443

c

τ (μs)e 212h 267 278 190 223 248 198 177 172 196 203 215 223 217 222

In deaerated CH2Cl2 solution (ca. 5 × 10−6 M). bIn PrCN, except where noted. cHighest-energy emission peak. dQuantum yield. eEmission lifetime. fRadiative rate constant, kr = Φ/τ. gNonradiative rate constant, knr = (1 − Φ)/τ. hIn MeTHF.

a

broader and slightly red-shifted in comparison with other sym-X derivatives (Figure 5). This is characteristic of 3LC emitting

Figure 5. Comparison of emission spectra of complexes sym-X and unsym-MeX (X = F, Cl, Br) in frozen matrices at 77 K.

states with an increased MLCT contribution9a and indicates a higher metal orbital involvement in the highest occupied molecular orbital (HOMO). Complexes unsym-X are all nonemissive in fluid solution at 298 K. However, they are strongly luminescent in frozen glasses at 77 K, giving rise to emission bands that are very similar in shape and energy to those found for the sym-X isomers and also to comparable lifetimes. This behavior is evidence that the emissions arise from essentially 3LC(ppy) excited states and also that nonradiative deactivation pathways become accessible at room temperature, reasonably because of the presence of low-lying, thermally populated, deactivating excited states. Since the deactivation processes appear to be independent of the ancillary ligands, it is reasonable that these states have 3 LMCT character [π(ppy) → dσ*(Pt)], as observed for mer[Pt(ppy)3]+.15a

Figure 4. Emission spectra of complexes sym-X and unsym-MeX in CH2Cl2 solution (ca. 1 × 10−5 M) at 298 K.

be attributed to its kr, which is significantly larger than that of the rest of the complexes of this series and points to a noticeably higher MLCT contribution to the emitting excited state. Additional support for this interpretation is provided by the emission spectrum of sym-F at 77 K, which is somewhat 7652


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Inorganic Chemistry The methyl derivatives unsym-MeX are strongly luminescent in solution at 298 K, with the exception of unsym-MeI, which is only weakly emissive. All of them show the same structured emission (Figure 4), which becomes sharper and undergoes a small rigidochromic shift at 77 K. A primarily 3LC(ppy) emitting state with some MLCT character is reasonable for these complexes, as previously proposed for unsym-MeCl.16 Remarkably, the variation of ligand X has generally minor effects on the emission efficiencies along this series, which contrasts with the behavior of the sym-X series. The maximum quantum yield corresponds to unsym-MeBr (Φ = 0.63) and only drops dramatically for unsym-MeI. The very low efficiency of the latter must be attributed to the high π-donating ability of the iodide ligand, which may give rise to low-lying, deactivating LLCT or LMCT states, leading to a large knr. Notwithstanding, the superior quantum yields of compounds unsym-MeX compared with the homologous sym-X indicate that deactivating excited states are much less accessible, which is reflected by their generally lower knr values. The variations in kr within this series are less pronounced than those found for sym-X complexes, although they follow a similar trend, excluding the iodide derivative (see the Computational section). As noted for sym-F, the emission spectrum of unsym-MeF at 77 K is noticeably broader and slightly red-shifted compared with the rest of the complexes of this series (Figure 5), which indicates a higher MLCT contribution to the emitting state; however, this does not translate into the highest quantum yield of the series because of a less favorable balance between kr and knr compared with the unsym-MeX homologues with X = Cl, Br, or TFA. Photochemistry of Complexes unsym-X. In the preceding section, we ascribed the lack of emission from complexes unsym-X in fluid solutions at 298 K to thermally accessible, deactivating LMCT states. This type of excited state is characteristic of complexes of highly electrophilic metal ions,25 like Pt(IV),26 Ni(III),27 or Au(III),28 and can lead to ligand dissociation and/or reduction of the metal center, because they involve the population of dσ* orbitals. We have also shown in a previous study that certain complexes of the type mer-[Pt(C^N)3]+ are not luminescent at 298 K in fluid solutions owing to the population of a reactive LMCT state, which eventually results in the isomerization to the facial isomer.15a In the light of these results, we hypothesized that complexes unsym-X could exhibit photoreactivity and performed irradiation experiments of CD2Cl2 solutions of the complexes with X = Cl−, Br−, OAc−, and TFA− using UVB light (36 W lamp, centered at 310 nm). The formation of photoproducts was followed by 1H NMR, and the results are summarized in Scheme 3. Additional details and 1H NMR spectra are given in the Supporting Information. The irradiation of unsym-Cl for 15 h at 298 K resulted in the formation of a new major product, which was identified as symCl. In a similar manner, unsym-TFA photoisomerized to symTFA, whereas the irradiation of unsym-OAc resulted in complex mixtures. Surprisingly, when the same photochemical conditions were applied to unsym-Br, the full consumption of the starting material was observed to give a mixture of unsym-Cl and sym-Cl as the major products, while sym-Br was not detected; continued irradiation of this mixture resulted in a higher conversion to sym-Cl. To further investigate this halide exchange, we also examined the photoreactivity of sym-Br. Irradiation of a solution of sym-Br in CD2Cl2 for 15 h resulted in a product mixture containing sym-Cl, as the major product, but also unsym-Cl, whereas longer irradiation times resulted in a

Scheme 3

higher conversion to the former isomer. The conversion of sym/unsym-Br into sym/unsym-Cl necessarily involves a reaction with the solvent, most probably the abstraction of a chlorine atom through a radical mechanism. In fact, the homolytic cleavage of PtIV−X bonds (X = OH−,29 Cl−,30 N3− 31) under photochemical conditions has been previously reported, resulting in Pt(III) fragments and X• radicals. In addition, Sanford’s group has found that the Pt−Pt bond in the dimer [PtIII(ppy)2Cl]2 can be homolytically broken under thermal or photochemical conditions to give the monomeric, radical species [PtIII(ppy)2Cl]•, which subsequently abstracts a chlorine atom from CH2Cl2 to yield a mixture of sym-Cl and unsym-Cl.21a It is thus reasonable that the irradiation of unsymCl or sym/unsym-Br leads to the population of a 3LMCT [π(ppy) or p(X) → dσ*(Pt)] state, leading to a homolytic cleavage of one of the Pt−X bonds. The resultant pentacoordinated Pt(III) species may then cause isomerization and/or abstract a chlorine atom from CD2Cl2. This would explain both the Br/Cl exchange upon irradiation of the bromide derivatives and also the detection of the photochemically unstable unsym-Cl during the irradiation of sym-Br. The fact that no chlorine abstraction takes place upon irradiation of unsym-TFA suggests a different isomerization mechanism, which may either involve a heterolytic Pt−O cleavage or the decoordination of one of the pyridyl moieties. Computational Study. To get additional insight into the factors that govern the excited-state behavior of the studied compounds, we performed density functional theory (DFT) and time-dependent (TD) DFT calculations at the B3LYP/(631G**+LANL2DZ) level considering solvent effects (CH2Cl2). Complexes sym-F, sym-Cl, sym-I, unsym-Cl, unsym-Br, unsymTFA, unsym-MeF, unsym-MeCl, and unsym-MeI were chosen as representative examples. The bond distances and angles of the optimized geometries (Tables S29 and S30) are in good agreement with the expected values in view of the available crystal structures. In particular, the calculations correctly reproduce the variations in the Pt−C bond distances depending on the ancillary ligand in trans and predict the shortest values for sym-F (2.017 Å) and unsym-MeF (2.004 Å). Figure 6 shows the frontier orbital energies for all the calculated complexes and a simplified, color-coded representation of their composition. Detailed compositions and spatial distributions are given in the Supporting Information. In general, the largest contributions to the majority of frontier orbitals come from π(ppy) or π*(ppy) 7653


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Figure 6. Frontier-orbital energy diagrams from DFT calculations in CH2Cl2 solution.

combinations with a high percentage of metal orbital contribution (up to 33%), which anticipates the existence of low-energy LMCT states. A comparison between the HOMO and LUMO distributions of sym-Cl and unsym-Cl (Figure 7)

orbitals, while contributions from metal or ancillary ligand orbitals vary significantly along the studied series. The HOMO in complexes sym-F, sym-Cl, unsym-Cl, unsym-TFA, unsymMeF, and unsym-MeCl is essentially a π(ppy)/d(Pt) combination with π-antibonding character along one (unsym complexes) or both (sym complexes) Pt−C bonds, in which the metal orbital involvement is generally higher than that found for the cationic complexes mer/fac-[Pt(ppy)3]+.15b Remarkably, complexes sym-F and unsym-MeF display the highest metal orbital contribution to the HOMO (21% or 17%, respectively) of the studied series, followed by sym-Cl (11%), unsym-MeCl (9%), unsym-Cl (8%), and unsym-TFA (7%). This fact must be a consequence of stronger π-bonding interactions between the metal and the ligands in sym-F and unsym-MeF, which, in turn, may be ascribed to two key factors: (a) the shorter Pt−C distances, and (b) a more effective d(Pt)-p(F) orbital overlap, because p(F) orbitals are more compact and similar in energy to d(Pt) orbitals in comparison to those of other ligands (see Figure S16 for a comparison between sym-F and sym-Cl MOs involved in Pt−X bonding). Occupied π(ppy)/d(Pt) orbitals in the rest of calculated derivatives lie below the HOMO. Contributions from ancillary ligand lone pairs to the HOMOs are generally in accord with their expected energies. Thus, while in sym-F there are negligible contributions from p(F) orbitals to the first four occupied MOs, the contributions from p(Cl) orbitals in sym-Cl are appreciably higher, and in sym-I the HOMO, HOMO−1, and HOMO−2 are almost purely iodine p-orbitals. An analogous increasing contribution from p(X) orbitals to the occupied MOs can also be observed for the calculated unsym-X (TFA < Cl < Br) and unsym-MeX (Cl < I) derivatives. The lowest unoccupied molecular orbital (LUMO) in sym-X derivatives is essentially a π*(ppy) orbital, with little metal orbital involvement; unoccupied MOs with high metal orbital contributions [π*(ppy)/dσ*(Pt); up to 46% Pt] lie at significantly higher energies in complex sym-F compared with sym-Cl and sym-I, which can be attributed to a stronger σbonding interaction between the metal and the ligands. The already noted shorter Pt−C bonds and more effective d(Pt)p(F) overlap may explain this observation. In contrast to sym-X, the LUMOs in unsym-X complexes are π*(ppy)/dσ*(Pt)

Figure 7. HOMO and LUMO isosurfaces (0.03 e bohr−3) for sym-Cl and unsym-Cl.

helps to understand their divergent behavior. The dσ*(Pt) orbitals in unsym-X are distributed along the N−Pt−X axis in the three calculated complexes, and hence their population may lead the dissociation of either Pt−N or Pt−X bond, which could initiate the photoisomerization reactions. Remarkably, the substitution of a chloride in unsym-Cl for a methyl ligand has a critical impact, since it pushes the unoccupied metal orbitals to high energies in unsym-MeCl; in fact, unoccupied 7654


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Figure 8. Spin density distributions (0.001 e bohr−3) for the optimized lowest triplet excited states.

LLCT/LMCT character [p(I) → π*(ppy)/dσ*(Pt)]; this state is expected to be highly distorted and poorly emissive, which would explain the lack of emission at 298 K. Given that sym-I is emissive in frozen glasses at 77 K and that its emission is very similar to those found for the rest of derivatives, its emitting state is expected to be an analogous 3LC(ppy) state, which should correspond to T2 (Supporting Information), whose energy (2.90 eV) is similar to the T1 excitation in the rest of derivatives. This result can be explained by the much more pronounced effect of temperature and medium rigidity on charge-transfer states in comparison with LC states, which can result in similar energies for LLCT and LC states at 77 K or even in inversion of their relative ordering.24,33 Complexes unsym-Cl, unsym-Br, and unsym-TFA possess 3LC/LMCT [π(ppy) → π*(ppy)/dσ*(Pt)] states with a significant LMCT character (>10%), which lie relatively close to the lowest triplet (excitation energies in the range of 3.01−3.44 eV). Therefore, the calculations corroborate that 3LMCT states in these derivatives are thermally accessible and therefore could cause nonradiative deactivation at 298 K and be responsible for the photoisomerization reactions. In contrast, similar states are appreciably higher in energy in sym-F (3.78 eV) and sym-Cl (3.53 eV), which explains their more efficient emissions at 298 K. Triplet excited states with LMCT contributions higher than 10% are not observed below 4.58 eV in complexes unsym-MeF, unsym-MeCl, and unsym-MeI, suggesting that they cannot contribute to thermal deactivation of the emitting state. The main difference among these derivatives is the presence of lowlying LLCT [p(I) → π*(ppy)] states in unsym-MeI, indicating that the population of 3LLCT and not 3LMCT states could be responsible of its low quantum yield at 298 K. A similar emission quenching caused by LLCT instead of LMCT states was recently proposed for gold(III) compounds, which has its origin in the large nonradiative constants associated with LLCT states.28b Further understanding of the emissive behavior of the studied complexes was gained by optimizing the geometry of the T1 state of sym-F, sym-Cl, unsym-MeF, unsym-MeCl, and unsym-MeI in CH2Cl2 solution. The calculated electronic energies relative to the ground state (adiabatic energy differences) are 2.81 eV (sym-F and sym-Cl; 441 nm), 2.83 eV (unsym-MeF), or 2.84 eV (unsym-MeCl and unsym-MeI; 436 nm), in agreement with the almost invariable experimental emission energies. The spin density distributions (Figure 8) match in all cases a π−π* excitation almost entirely localized on one of the ppy ligands, reflecting the predominantly LC character of the emitting T1 state. However, there is a variable contribution of platinum orbitals, as shown by the calculated natural spin densities on this atom, which decrease along the sequences sym-F (0.0416) > sym-Cl (0.0225) and unsym-MeF (0.0497) > unsym-MeCl (0.0322) > unsym-MeI (0.0267).

π*(ppy)/dσ*(Pt) orbitals in unsym-MeX derivatives are significantly higher than in the corresponding sym-X complexes. Reasonably, this effect is caused by the strong σ-donating ability of the methyl ligand. Excitation energies at the ground-state geometries were calculated by TDDFT in CH2Cl2 solution. The data for selected singlet and triplet excitations are given in the Supporting Information, including the percentage of MLCT or LMCT character, which was estimated from the metalorbital contribution to the involved orbitals.15b,32 The predicted singlet excitation energies and oscillator strengths are in good qualitative agreement with the experimental absorption spectra in all the studied cases. The lowest-energy singlet excitation with a significant oscillator strength in sym-F (S1) or sym-Cl (S1) is a primarily LC(ppy) transition with some MLCT character, which matches the experimental intense absorption in the range of 335−338 nm. Notably, this transition has a much higher MLCT character in sym-F (17.0% vs 4.9% in symCl) and also a lower energy, in agreement with the higher HOMO energy and the increased metal-orbital contribution to this orbital, which also explains the red shift of the lowestenergy band in the experimental absorption spectrum. In the case of sym-I, the calculations predict several low-energy LLCT/LMCT [p(I) → π*(ppy)/dσ*(Pt)] transitions with low oscillator strengths, which are in good correlation with the long tail observed in the experimental absorption spectrum, while the stronger absorption at ∼339 nm corresponds to LC/ LLCT/LMCT admixtures [π(ppy)/p(I) → π*(ppy)/dσ*(Pt)]. In unsym-X (X = Cl−, Br−, TFA−), the lowest-energy excitations correspond to mixed LC/LMCT [π(ppy) → π*(ppy)/ dσ*(Pt)] transitions with variable LLCT character [π(ppy) → π*(ppy′)]. The significant LMCT character of these transitions (up to 25%) is a consequence of the previously noted lower dσ*(Pt) energies in these derivatives relative to sym-X. The lowest-energy singlet excitations in unsym-MeF and unsym-MeCl correspond to LC/MLCT transitions with up to 14% or 7% of MLCT character, respectively. However, in the case of unsym-MeI, the lowest singlets correspond to LLCT [p(I) → π*(ppy)] transitions with low oscillator strengths that are not present in unsym-MeF or unsym-MeCl and are responsible for the long tail observed in the experimental absorption spectrum; more intense excitations at higher energies in unsym-MeI correspond to LC(ppy) or mixed LC/ LLCT/MLCT [π(ppy)/p(I)/d(Pt) → π*(ppy)] transitions. With the exception of sym-I, the TDDFT results predict a primarily 3LC(ppy) state as the lowest triplet (T1) in the calculated complexes, with variable degrees of MLCT or LMCT character and similar energies (2.90−2.97 eV above S0), in agreement with the very similar experimental emissions. It is worth to mention the relatively high MLCT character found in the first four triplets of sym-F (6.0−8.2%) and unsym-MeF (6.7−10.1%). The lowest-energy triplet in sym-I has a mixed 7655


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Inorganic Chemistry Table 4. Crystallographic Data formula fw T (K) l (Å) cryst syst space group a (Å) b (Å) c (Å) a (deg) b (deg) g (deg) V (Å3) Z rcalcd (Mg m−3) m (mm−1) R1a wR2b

sym-OTf

sym-Br·CH2Cl2

sym-TFA

unsym-MeF

unsym-MeBr

C24H16F6N2O6PtS2 801.60 100(2) 0.710 73 monoclinic C2/c 9.7777(9) 14.5379(13) 18.5728(19) 90 103.671(3) 90 2565.3(4) 4 2.076 5.722 0.0132 0.0316

C23H18Br2Cl2N2Pt 748.20 100(2) 0.710 73 monoclinic P21/c 12.7573(11) 13.6342(12) 13.0045(11) 90 97.128(3) 90 2244.5(3) 4 2.214 10.064 0.0324 0.0612

C26H16F6N2O4Pt 729.50 240(2) 0.710 73 triclinic Pn 9.1313(6) 11.5755(7 13.0600(8) 100.955(2) 107.271(3) 104.593(2) 1222.16(13) 2 1.982 5.824 0.0490 0.1301

C23H19FN2Pt 537.49 100(2) 0.710 73 orthorhombic Fdd2 24.0111(11) 43.5461(19) 6.9271(3) 90 90 90 7242.9(6) 16 1.972 7.769 0.0171 0.0268

C23H19BrN2Pt 598.40 100(2) 0.710 73 monoclinic P21/c 8.7809(8) 13.9674(12) 15.6904(15) 90 98.541(3) 90 1903.0(3) 4 2.089 9.483 0.0223 0.0432

a R1 = S||F0| − |Fc||/S|F0| for reflections with I > 2s(I). bwR2 = [S[w(F02 − Fc2)2]/S[w(F02)2]0.5 for all reflections; w−1 = S2(F2) + (aP)2 + bP, where P = (2Fc2 + F02)/3, and a and b are constants set by the program.

deactivate their emissions in solution at 298 K and are also responsible for their photoisomerization reactions. However, in complexes unsym-MeX, the strong σ-donating ability of the methyl ligand contributes to raise dσ*(Pt) orbitals and hence the energy of LMCT states; as a result, nonradiative deactivation in these derivatives is generally less important than in sym-X and unsym-X, and quantum yields are higher. In short, this study provides substantial insight into the factors that determine the excited-state properties of biscyclometalated Pt(IV) complexes, demonstrating that variations in the nature of ancillary ligands and/or the arrangement of the cyclometalated ligands can have a profound impact on their emission efficiencies. The present results may thus constitute the basis for future developments in luminescent Pt(IV) complexes.

Interestingly, these trends can be connected with those found for the experimental kr values at room temperature, excluding unsym-MeI. Thus, a higher spin density on the metal generally means a higher kr, which thus can be associated with a higher metal orbital involvement in T1 and also a higher MLCT character of this state. This is most clearly seen for sym-F and unsym-MeF, which display the highest MLCT character in the lowest-energy vertical excitations due to the high metal-orbital involvement in their HOMO, ultimately resulting in a higher MLCT character in the emitting T1 state and a higher kr. The connection between MLCT character in mixed LC/MLCT excited states and radiative rate constants has been previously noted10a and becomes a crucial factor in essentially 3LC emitters. The calculated natural spin density on the metal in complex unsym-MeI is much lower than expected in view of its large kr; however, small contributions of iodine orbitals to T1 (natural spin density on I: 0.0319) may contribute to increase kr because of the strong spin−orbit effect induced by this heavy atom.

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EXPERIMENTAL SECTION

General Considerations, Materials, and Instrumentation. Unless otherwise noted, preparations were performed at room temperature under atmospheric conditions. Synthesis-grade solvents were obtained from commercial sources and used without further purification, except for CH2Cl2, which was distilled over CaH2. The compounds sym-Cl,12 cis-[Pt(ppy)2],20b unsym-X (X = Cl, Br, OAc, TFA),21 and unsym-MeCl16 were prepared following published procedures. All other reagents were obtained from commercial sources. NMR spectra were recorded on Bruker Avance 300 or 400 spectrometers at 298 K. Chemical shifts are referred to residual signals of nondeuterated solvent (1H, 13C{1H}) or external CFCl3 (19F{1H}). The number of solvation H2O or CH2Cl2 molecules was calculated from the integral of the 1H NMR signal, taking into account the solvent blank. The 13C{1H} NMR spectra of complexes sym-X (X = F, Br, I and TFA) could not be registered because of their very low solubility in all common deuterated solvents. Elemental analyses were performed with Carlo Erba 1106 and LECO CHNS-932 microanalyzers. UV−vis absorption spectra were recorded on a PerkinElmer Lambda 750S spectrophotometer. Excitation and emission spectra were recorded on a Jobin Yvon Fluorolog 3−22 spectrofluorometer with a 450 W xenon lamp, double-grating monochromators, and a TBX-04 photomultiplier. Measurements were performed in a right angle configuration using 10 mm quartz fluorescence cells for solutions at 298 K or 5 mm quartz NMR tubes for frozen glasses at 77 K. A

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CONCLUSIONS The luminescence of bis-cyclometalated Pt(IV) complexes featuring either a C2-symmetrical or unsymmetrical {Pt(ppy)2} unit has been systematically examined. Although all the studied complexes exhibit very similar 3LC(ppy) emitting states, the nature of the ancillary ligands and the arrangement of the cyclometalated ppy ligands exert a critical influence on their emission efficiencies. Within the sym-X series, complex sym-F exhibits the highest quantum yield, because the high MLCT contribution to the emitting state leads to a high kr value, while deactivating LMCT states lie at high energies. In the rest of sym-X derivatives, the MLCT contribution to the emitting state decreases, which has an adverse effect on kr, and nonradiative deactivation becomes more important, mainly because the energies of deactivating states of LMCT or LLCT character decrease, resulting in less efficient emissions at 298 K. The unsymmetrical arrangement of the ppy ligands leads to significantly lower dσ*(Pt) orbital energies in derivatives unsym-X, resulting in low-lying LMCT states that totally 7656


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Inorganic Chemistry

0.9 Hz, JHPt = 33 Hz, 2 H). 19F{1H} NMR (282.4 MHz, CD2Cl2): δ −234.6 (s with satellites, JFPt = 85 Hz). sym-[Pt(ppy)2Br2] (sym-Br). To a solution of sym-OTf (102 mg, 0.13 mmol) in acetone (15 mL) was added NaBr (76 mg, 0.73 mmol), and the resultant suspension was stirred for 90 min. The solvent was then removed under reduced pressure, and the residue was treated with water (3 × 30 mL) to extract the NaOTf and excess NaBr. During this process, the suspension was centrifuged to separate the fine white precipitate from the supernatant solution. Finally, the solid was stirred in MeOH (10 mL), collected by filtration, washed with Et2O (5 mL), and vacuum-dried to give sym-Br. Yield: 65 mg, 78%. Anal. Calcd for C22H16Br2N2Pt: C, 39.84; H, 2.43; N, 4.22. Found: C, 39.94; H, 2.61; N, 4.40%. 1H NMR (400.9 MHz, CD2Cl2): δ 10.15 (d with satellites, JHH = 5.6 Hz, JHPt = 29 Hz, 2H), 8.13 (td, J = 8.0, 1.2 Hz, 2H), 8.05−8.02 (m, 2H), 7.66 (dd, J = 7.6 Hz, 1.2 Hz, 2H), 7.52 (m, 2H), 7.14 (td, J = 7.6, 0.8 Hz, 2H), 6.96 (m, 2H), 6.01 (dd with satellites, JHH = 8.0, 0.8 Hz, JHPt = 33 Hz, 2H). sym-[Pt(ppy)2I2] (sym-I). To a solution of sym-OTf (100 mg, 0.12 mmol) in acetone (12 mL) was added NaI (40 mg, 0.27 mmol), and the mixture was stirred for 1 h. A pale yellow precipitate gradually formed, which was filtered off, washed with acetone (2 × 5 mL) and Et2O (5 mL), and vacuum-dried to give sym-I. Yield: 74 mg, 78%. Anal. Calcd for C22H16I2N2Pt: C, 34.89; H, 2.13; N, 3.70. Found: C, 34.85; H, 2.22; N, 3.85%. 1H NMR (400.9 MHz, CD2Cl2): δ 10.44 (ddd with satellites, JHH = 6.0, 1.2, 0.4 Hz, JHPt = 31 Hz, 2H), 8.12 (m, 2H), 8.05−8.02 (m, 2H), 7.63 (dd, J = 7.8, 1.2 Hz, 2H), 7.45 (m, 2H), 7.11 (td, J = 7.6, 1.2 Hz, 2H), 6.97−6.93 (m, 2H), 6.01 (dd with satellites, JHH = 8.0, 0.8 Hz, JHPt = 32 Hz, 2H). sym-[Pt(ppy)2(OAc)2] (sym-OAc). To a suspension of sym-Cl (153 mg, 0.27 mmol) in 1,2-dichloroethane (50 mL) was added AgOAc (100 mg, 0.60 mmol), and the mixture was stirred at 90 °C in the dark for 4 h. The resultant suspension was allowed to cool to room temperature and filtered through diatomaceous earth to remove the precipitate of AgCl. The solvent was removed under reduced pressure, and the residue was treated with CH2Cl2 (12 mL) and n-pentane (40 mL), whereupon a white solid precipitated, which was filtered off, washed with Et2O (5 mL), and vacuum-dried to give sym-OAc. Yield: 61 mg, 37%. Anal. Calcd for C26H22N2O4Pt: C, 50.24; H, 3.57; N, 4.51. Found: C, 50.11; H, 3.83; N, 4.41%. 1H NMR (400.9 MHz, CD2Cl2): δ 9.56 (dd with satellites, JHH = 6.0, 1.4 Hz, JHPt = 27 Hz, 2H), 8.11 (m, 2H), 7.95 (m, 2H), 7.59 (dd, J = 7.7, 1.2 Hz, 2H), 7.54 (m, 2H), 7.11 (td, J = 7.5, 1.0 Hz, 2H), 6.88 (m, 2H), 5.93 (dd with satellites, JHH = 7.9, 0.8 Hz, JHPt = 31 Hz, 2H), 1.81 (s, 6H). 13C{1H} NMR (100.8 MHz, CD2Cl2): δ 175.7 (C), 164.8 (C), 148.9 (CH), 142.3 (C), 141.4 (CH), 135.1 (C), 131.0 (JCPt = 41 Hz, CH), 128.2 (JCPt = 28 Hz, CH), 126.0 (CH), 124.8 (JCPt = 31 Hz, CH), 123.1 (JCPt = 27 Hz, CH), 120.2 (JCPt = 35 Hz, CH), 25.0 (JCPt = 15 Hz, CH3). sym-[Pt(ppy)2(TFA)2] (sym-TFA). To a solution of sym-OTf (104 mg, 0.13 mmol) in acetone (20 mL) was added CF3COONa (60 mg, 0.44 mmol), and the mixture was stirred for 5 h. The solvent was removed under reduced pressure, and the residue was treated with CH2Cl2 (40 mL) to give a white suspension, which was stirred for 5 min and filtered through diatomaceous earth to remove the precipitate of NaOTf. The filtrate was concentrated under reduced pressure (5 mL) and Et2O (50 mL) was added, whereupon a white solid precipitated, which was filtered off, washed with Et2O (5 mL) and vacuum-dried to give sym-TFA. Yield: 65 mg, 69%. Anal. Calcd for C26H16F6N2O4Pt: C, 42.81; H, 2.21; N, 3.84. Found: C, 42.80; H, 2.31; N, 4.06%. 1H NMR (400.9 MHz, CD2Cl2): δ 9.42 (ddd with satellites, JHH = 5.9, 1.4, 0.6 Hz, JHPt = 26 Hz, 2H), 8.20 (m, 2H), 8.03 (m, 2H), 7.65−7.60 (m, 4H), 7.20 (m, 2H), 6.95 (m, 2H), 5.97 (dd with satellites, JHH = 8.0, 0.9 Hz, JHPt = 33 Hz, 2H). 19F{1H} NMR (188.3 MHz, CD2Cl2): δ −75.9 (s). unsym-[Pt(ppy)2Me(OTf)] (unsym-MeOTf). In a flame-dried Schlenk flask under an N2 atmosphere were placed unsym-MeCl (150 mg, 0.27 mmol), AgOTf (77 mg, 0.30 mmol), molecular sieves (4 Å) and CH2Cl2 (50 mL). The resultant suspension was stirred for 16 h in the dark and then filtered through diatomaceous earth. The filtrate was concentrated under reduced pressure (8 mL) and npentane (50 mL) was added, whereupon a white solid precipitated,

liquid nitrogen Dewar with quartz windows was employed for the lowtemperature measurements. Solutions of the samples were previously degassed by bubbling argon for 30 min. Emission lifetimes (τ) were measured using either the Fluorolog’s FL-1040 phosphorimeter accessory (τ > 10 μs) or an IBH FluoroHub TCSPC controller and a NanoLED pulse diode excitation source (τ < 10 μs); the estimated uncertainty is ±10% or better. Emission quantum yields (Φ) were measured using a Hamamatsu C11347 Absolute PL Quantum Yield Spectrometer; the estimated uncertainty is ±5% or better. Quantum yields lower than 0.01 were determined by the relative method,34 using sym-Cl (Φ = 0.114) as standard. X-ray Structure Determinations. Crystals of sym-OTf, sym-Br· CH2Cl2, sym-TFA, unsym-MeF, and unsym-MeBr suitable for X-ray diffraction studies were obtained by slow evaporation from a CH2Cl2 solution (sym-OTf) or liquid−liquid diffusion from CH2Cl2/Et2O (rest of complexes). The data were collected on a Bruker D8 QUEST diffractometer with monochromated Mo Kα radiation performing ω scans (sym-OTf, unsym-MeF) or φ and ω scans (rest of complexes). The structures were solved by direct methods and refined anisotropically on F2 using the program SHELXL-2013 or SHELXL-2014.35 Methyl hydrogens were included as part of rigid idealized methyl groups allowed to rotate but not tip; other hydrogens were included using a riding model. Numerical details are presented in Table 4. Complete crystallographic data are included in the Supporting Information in CIF format and have also been deposited with the Cambridge Crystallographic Data Centre (CCDC 1468744−1468746, 1468749, 1468750). Special features of ref inement: The two TFA ligands in sym-TFA are disordered; in one of them, the CF3 group is disordered over two sites (ca. 51:49), while in the other one the disorder affects the CF3 group and the uncoordinated oxygen (ca. 52:48%). The disordered atoms were refined as isotropic. The structure of unsym-MeBr showed disordered methyl and bromide ligands located on each other’s sites with relative occupancies of ca. 90:10%. We interpret this disorder as the result of the superimposition of both enantiomers of the complex on the same crystallographic position. The assignment of the metalated carbons and the nitrogens corresponds to the molecule with the major occupancy factor and were made on the basis of bond lengths and chemical information. sym-[Pt(ppy)2(OTf)2] (sym-OTf). To a suspension of sym-Cl (800 mg, 1.39 mmol) in acetone (120 mL) was added AgOTf (788 mg, 3.06 mmol), and the mixture was refluxed in the dark for 4 h. The resulting suspension was filtered through diatomaceous earth to remove the precipitate of AgCl. The solvent was removed under reduced pressure, and the residue was subjected to two successive dissolution/ evaporation cycles using CH2Cl2 (15 mL) to ensure the complete removal of acetone. Treatment with CH2Cl2 (20 mL) and addition of Et2O (80 mL) led to the formation of a white precipitate, which was filtered off, washed with Et2O (10 mL), and vacuum-dried to give symOTf. Yield: 903 mg, 81%. Anal. Calcd for C24H16F6N2O6PtS2: C, 35.96; H, 2.01; N, 3.50; S, 8.00. Found: C, 35.76; H, 2.02; N, 3.48; S, 8.08%. 1H NMR (400.9 MHz, CD2Cl2): δ 9.18 (dd with satellites, JHH = 6.0, 0.9 Hz, JHPt = 24 Hz, 2H), 8.28 (m, 2H), 8.08 (m, 2H), 7.71− 7.64 (m, 4H), 7.27 (td, J = 7.5, 1.0 Hz, 2H), 6.98 (m, 2H), 5.98 (dd with satellites, JHH = 8.1, 0.9 Hz, JHPt = 33 Hz, 2H). 19F{1H} NMR (282.4 MHz, CD2Cl2): δ −78.7 (s). The 13C{1H} NMR spectrum could not be registered due to the very poor solubility of this complex in noncoordinating solvents (CD2Cl2, CDCl3). sym-[Pt(ppy)2F2] (sym-F). A 1 M solution of tetrabutylammonium fluoride in THF (1.50 mL, 1.50 mmol) was added to a suspension of sym-OTf (200 mg, 0.25 mmol) in MeOH (5 mL), and the mixture was stirred for 4 h in the dark. The resultant solution was concentrated under reduced pressure (2 mL), and Et2O (70 mL) was added, whereupon an off-white solid precipitated, which was filtered off, washed with CH2Cl2 (2 × 5 mL), and vacuum-dried to give sym-F· 2H2O. Yield: 62 mg, 46%. Anal. Calcd for C22H20F2N2O2Pt: C, 45.76; H, 3.49; N, 4.85. Found: C, 45.71; H, 3.80; N, 4.80%. 1H NMR (300.1 MHz, CD2Cl2): δ 9.38 (d with satellites, JHH = 5.7 Hz, JHPt = 25 Hz, 2H), 8.15 (m, 2H), 8.04−7.98 (m, 2H), 7.66−7.56 (m, 4H), 7.15 (td, J = 7.5, 1.5 Hz, 2H), 6.92 (m, 2H), 6.09 (dd with satellites, JHH = 7.8, 7657


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Inorganic Chemistry which was filtered off, washed with n-pentane (5 mL) and vacuumdried to give unsym-MeOTf. Yield: 142 mg, 79%. Anal. Calcd for C24H19F3N2O3PtS: C, 43.18; H, 2.87; N, 4.20; S, 4.80. Found: C, 43.19; H, 2.98; N, 4.17; S, 4.80. 1H NMR (400.9 MHz, CD2Cl2): δ 9.12 (ddd with satellites, JHH = 5.5, 1.5, 0.9 Hz, JHPt ≈ 9 Hz, 1H), 8.16−8.08 (m, 2H), 7.99 (d, J = 8.2 Hz, 1H), 7.88−7.75 (m, 3H), 7.69−7.57 (m, 3H), 7.49−7.37 (m, 2H), 7.08 (m, 2H), 6.82 (m, 1H), 6.37 (dd with satellites, JHH = 8.0, 1.1 Hz, JHPt = 63 Hz, 1H), 1.23 (s with satellites, JHPt = 65 Hz, 3H). 13C{1H} NMR (100.8 MHz, CD2Cl2): δ 162.8 (C), 160.3 (C), 147.7 (CH), 146.7 (CH), 143.8 (C), 142.3 (C), 140.6 (CH), 140.0 (CH), 132.7 (JCPt = 63 Hz, CH), 131.8 (JCPt = 55 Hz, CH), 131.4 (JCPt = 76 Hz, CH), 131.0 (JCPt = 36 Hz, CH), 127.5 (C), 126.1 (CH), 125.6 (CH), 125.3 (CH), 125.2 (CH), 125.1 (CH), 123.8 (CH), 120.7 (JCPt = 16 Hz, CH), 120.3 (JCPt = 14 Hz, CH), −0.83 (JCPt = 623 Hz, CH3). 19F{1H} NMR (282.4 MHz, CD2Cl2): δ −78.9 (s). unsym-[Pt(ppy)2MeF] (unsym-MeF). To a solution of unsymMeCl (105 mg, 0.19 mmol) in CH2Cl2 (20 mL) was added AgF (96 mg, 0.76 mmol). The resultant suspension was stirred in the dark for 15 h and then filtered through diatomaceous earth. A second addition of AgF (96 mg, 0.76 mmol) was made to the filtrate, the suspension was stirred in the dark for 5 h and then filtered through diatomaceous earth. The filtrate was concentrated under reduced pressure (10 mL) and n-pentane (40 mL) was added, whereupon a white solid precipitated, which was filtered off, washed with Et2O (2 × 5 mL) and vacuum-dried to give unsym-MeF·0.5H2O. Yield: 65 mg, 64%. Anal. Calcd for C23H20FO0.5N2Pt: C, 50.55; H, 3.69; N, 5.13. Found: C, 50.49; H, 3.44; N, 5.24%. 1H NMR (400.9 MHz, CD2Cl2): δ 9.19 (d with satellites, JHH = 4.9 Hz, JHPt ≈ 9 Hz, 1H), 8.11−8.04 (m, 2H), 7.96 (d, J = 8.1 Hz, 1H), 7.87−7.75 (m, 3H), 7.66−7.58 (m, 2H), 7.51 (m, 1H), 7.42 (m, 1H), 7.35 (m, 1H), 7.06−7.00 (m, 2H), 6.82 (m, 1H), 6.52 (ddd with satellites, JHH = 7.8, 2.7, 1.1 Hz, JHPt = 52 Hz, 1H), 1.10 (d with satellites, JHF = 7.7 Hz, JHPt = 67 Hz, 3H). 13C{1H} NMR (100.8 MHz, CD2Cl2): δ 162.4 (C), 161.9 (C), 146.8 (CH), 146.2 (CH), 144.0 (C), 143.5 (C), 141.5 (C), 140.1 (CH), 139.3 (CH), 134.0 (JCPt = 54 Hz, CH), 134.0 (JCPt = 55 Hz, CH), 132.3 (C), 131.5 (JCPt = 56 Hz, CH), 131.0 (CH), 130.9 (CH), 130.9 (CH), 125.3 (JCPt = 32 Hz, CH), 125.2 (CH), 124.8 (JCPt = 36 Hz, CH), 124.3 (CH), 123.9 (CH), 123.6 (CH), 120.2 (JCPt = 14 Hz, CH), − 2.1 (d with satellites, JCF = 6.5 Hz, JCPt = 648 Hz, CH3). 19F{1H} NMR (282.4 MHz, CD2Cl2): δ −257.5 (br). unsym-[Pt(ppy)2MeBr] (unsym-MeBr). To a solution of unsymMeOTf (100 mg, 0.15 mmol) in acetone (20 mL) was added NaBr (46 mg, 0.45 mmol) and the mixture was stirred for 4 h. The solvent was removed under reduced pressure and the residue was treated with CH2Cl2 (30 mL). The resultant suspension was filtered through diatomaceous earth, and the filtrate was concentrated (10 mL). The addition of n-pentane (40 mL) led to the precipitation of a beige solid, which was filtered off, washed with Et2O (5 mL), and vacuum-dried to give unsym-MeBr. Yield: 73 mg, 82%. Anal. Calcd for C23H19BrN2Pt: C, 46.16; H, 3.20; N, 4.68. Found: C, 46.10; H, 3.20; N, 4.79%. 1H NMR (300.1 MHz, CD2Cl2): δ 9.93 (dt with satellites, JHH = 5.4, 1.1 Hz, JHPt ≈ 10 Hz, 1H), 8.10−8.04 (m, 2H), 7.95 (d, J = 8.1 Hz, 1H), 7.83−7.71 (m, 3H), 7.66 (m, 1H), 7.58−7.52 (m, 2H), 7.44 (m, 1H), 7.32 (m, 1H), 7.08−6.97 (m, 2H), 6.86 (m, 1H), 6.41 (dd with satellites, JHH = 7.8, 1.0 Hz, JHPt = 54 Hz, 1H), 1.20 (s with satellites, JHPt = 69 Hz, 3H). 13C{1H} NMR (75.45 MHz, CD2Cl2): δ 162.4 (C), 162.1 (C), 149.6 (CH), 146.4 (CH), 143.6 (C), 142.6 (C), 142.4 (C), 140.9 (C), 139.8 (CH), 139.0 (CH), 131.8 (JCPt = 55 Hz, CH), 131.7 (JCPt = 58 Hz, CH), 131.4 (JCPt = 65 Hz, CH), 130.7 (JCPt = 37 Hz, CH), 125.3 (CH), 125.1 (CH), 124.8 (CH), 124.8 (CH), 124.6 (CH), 123.4 (CH), 120.3 (JCPt = 14 Hz, CH), −4.0 (JCPt = 618 Hz, CH3). unsym-[Pt(ppy)2MeI] (unsym-MeI). This complex was obtained as a pale orange solid following the procedure described for unsymMeBr, from unsym-MeOTf (75 mg, 0.11 mmol) and NaI (30 mg, 0.20 mmol). Yield: 46 mg, 64%. Anal. Calcd for C23H19IN2Pt: C, 42.80; H, 2.97; N, 4.34. Found: C, 42.63; H, 3.08; N, 4.46%. 1H NMR (400.9 MHz, CD2Cl2): δ 10.23 (dt with satellites, JHH = 5.5, 1.1 Hz, JHPt ≈ 10 Hz, 1H), 8.08−8.03 (m, 2H), 7.94 (d, J = 8.4 Hz, 1H), 7.82 (dd, J =

7.9, 1.5 Hz, 1H), 7.79−7.59 (m, 4H), 7.51 (m, 1H), 7.44 (m, 1H), 7.29 (m, 1H), 7.05 (m, 1H), 6.97 (m, 1H), 6.88 (m, 1H), 6.25 (dd with satellites, JHH = 7.9, 1.0 Hz, JHPt = 54 Hz, 1H), 1.34 (s with satellites, JHPt = 70 Hz, 3H). 13C{1H} NMR (75.45 MHz, CD2Cl2): δ 162.3 (C), 162.2 (C), 152.5 (CH), 146.8 (CH), 142.4 (CH), 142.1 (C), 141.7 (C), 141.0 (C), 139.6 (CH), 138.8 (CH), 131.8 (JCPt = 58 Hz, CH), 131.3 (JCPt = 64 Hz, CH), 131.0 (JCPt = 72 Hz, CH), 130.6 (JCPt = 58 Hz, CH), 125.3 (CH), 125.2 (CH), 125.0 (CH), 124.7 (CH), 123.3 (CH), 120.5 (JCPt = 14 Hz, CH), 120.4 (JCPt = 14 Hz, CH), −5.9 (JCPt = 610 Hz, CH3). unsym-[Pt(ppy)2Me(TFA)] (unsym-MeTFA). This complex was obtained as a white solid following the procedure described for unsymMeBr, from unsym-MeOTf (100 mg, 0.15 mmol) and CF3COONa (61 mg, 0.45 mmol). Yield: 58 mg, 62%. Anal. Calcd for C25H19F3N2O2Pt: C, 47.55; H, 3.03; N, 4.44. Found: C, 47.52; H, 3.11; N, 4.52%. 1H NMR (400.9 MHz, CD2Cl2): δ 9.00 (dt with satellites, JHH = 5.4, 1.2 Hz, JHPt ≈ 8 Hz, 1H), 8.12−8.07 (m, 2H), 7.94 (d, J = 8.1 Hz, 1H), 7.88−7.75 (m, 3H), 7.66−7.56 (m, 3H), 7.43 (m, 1H), 7.35 (m, 1H), 7.07−7.00 (m, 2H), 6.83 (m, 1H), 6.38 (dd with satellites, JHH = 7.9, 1.0 Hz, JHPt = 55 Hz, 1H), 1.16 (s with satellites, JHPt = 66 Hz, 3H). 13C{1H} NMR (100.8 MHz, CD2Cl2): δ 162.6 (C), 161.7 (C), 147.5 (CH), 146.9 (CH), 143.4 (C), 143.0 (C), 140.8 (C), 140.3 (CH), 139.3 (CH), 132.9 (JCPt = 58 Hz, CH), 132.4 (C), 131.4 (JCPt = 55 Hz, CH), 131.1 (JCPt = 64 Hz, CH), 130.8 (JCPt = 36 Hz, CH), 125.3 (CH), 125.1 (CH), 124.9 (CH), 124.4 (CH), 123.2 (CH), 120.5 (JCPt = 16 Hz, CH), 119.9 (JCPt = 14 Hz, CH), −1.8 (JCPt = 634 Hz, CH3). 19F{1H} NMR (282.4 MHz, CD2Cl2): δ −75.5 (s). Computational Methods. DFT calculations were performed with the Gaussian 09 package,36 using the hybrid B3LYP functional37 together with the 6-31G**38 basis set for the light atoms and the LANL2DZ39 basis set and its associated relativistic effective core potential for Pt and I. All geometry optimizations were performed without symmetry restrictions, using “tight” convergence criteria and “ultrafine” integration grid. Vertical excitation energies were obtained from TDDFT calculations at the ground-state optimized geometries. Triplet-state geometry optimizations were performed following a twostep strategy.40 Initially, TDDFT optimizations of the two lowest triplets were attempted starting from the ground-state geometry; the resulting lowest-energy geometry was then subjected to spinunrestricted DFT (UB3LYP) optimizations setting a triplet multiplicity. The solvent effect (CH2Cl2) was accounted for in all cases by using the integral equation formalism variant of the polarizable continuum solvation model.41 All the optimized structures were confirmed as minima on the potential energy surface by performing frequency calculations (zero imaginary frequencies). Natural spin densities were obtained from natural population analyses using the NBO 5.9 program.42

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01100. Details on the photoreactions of unsym-X, 1H NMR spectra of new compounds, excitation and emission spectra, computational data (PDF) Crystallographic data (CIF)

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Corresponding Author

*E-Mail: [email protected]. Phone: +34 868887097. Fax: +34 868884148. Notes

The authors declare no competing financial interest. 7658


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Inorganic Chemistry

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ACKNOWLEDGMENTS This work was supported by Ministerio de Economiá y Competitividad, Spain (Grant No. CTQ2011-24016, with FEDER support), Ministerio de Educación, Cultura y Deporte, Spain (FPU Grant to F. J), Fundación Séneca (Grant No. 19890/GERM/15) and Universidad de Murcia.

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