Phenylbenzothiazole-Based Platinum(II) and Diplatinum(II) and (III) Complexes with Pyrazolate Groups: Optical Properties and Photocatalysis

Based on 2-phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me2N-pbt), mononuclear [Pt(pbt)(R′2-pzH)2]PF6 (R′2-pzH = pzH 1a, 3,5-Me2pzH 1b, 3,5-iPr2pzH 1c) and diplatinum (PtII–PtII) [Pt(pbt)(μ-R′2pz)]2 (R′2-pz = pz 2a, 3,5-Me2pz 2b, 3,5-iPr2pz 2c) and [Pt(Me2N-pbt)(μ-pz)]2 (3a) complexes have been prepared. In the presence of sunlight, 2a and 3a evolve, in CHCl3 solution, to form the PtIII–PtIII complexes [Pt(R-pbt)(μ-pz)Cl]2 (R = H 4a, NMe25a). Experimental and computational studies reveal the negligible influence of the pyrazole or pyrazolate ligands on the optical properties of 1a–c and 2a,b, which exhibit a typical 3IL/3MLCT emission, whereas in 2c the emission has some 3MMLCT contribution. 3a displays unusual dual, fluorescence (1ILCT or 1MLCT/1LC), and phosphorescence (3ILCT) emissions depending on the excitation wavelength. The phosphorescence is lost in aerated solutions due to sensitization of 3O2 and formation of 1O2, whose determined quantum yield is also wavelength dependent. The phosphorescence can be reversibly photoinduced (365 nm, ∼ 15 min) in oxygenated THF and DMSO solutions. In 4a and 5a, the lowest electronic transitions (S1–S3) have mixed characters (LMMCT/LXCT/L’XCT 4a and LMMCT/LXCT/ILCT 5a) and they are weakly emissive in rigid media. The 1O2 generation property of complex 3a is successfully used for the photooxidation of p-bromothioanisol showing its potential application toward photocatalysis.


■ INTRODUCTION
Luminescent transition-metal compounds have attracted considerable attention owing to their wide range of applicability, such as emitters for light-emitting devices, optoelectronic materials, bioimagen probes, sensors, and photocatalysis. 1Among these complexes, cyclometalated platinum complexes are considered one of the most promising materials due to their rich and tunable excited-state properties, which can be finetuned by molecular design. 2Thus, mononuclear Pt II complexes exhibit highly efficient triplet-state phosphorescence attributed to 3 LC( 3 ππ), 3 MLCT, or 3 LC/ 3 MLCT and 3 LLCT/ 3 MLCT mixtures, depending on the cyclometalating and ancillary ligands.A remarkable feature of these complexes is their propensity to develop, in the ground and/or upon excitation, noncovalent intermolecular platinum−platinum contacts through the interaction of the filled 5d z 2 orbitals and ππ stacking interactions, particularly favored by the coordination of planar chelating aromatic ligands.These interactions are accompanied by simultaneous changes in the spectroscopic and optical properties, such as assembly-induced metal−metal−to ligand charge-transfer (MMLCT) transitions, which appear at lower energies than the corresponding monomers, and may be utilized to achieve single-doped white OLEDs 3 or stimuli-responsive functional materials. 4Many of these systems have demonstrated to be solid-state low-red or near-infrared (NIR) emitters, with attractive applications in OLEDs 5 or biological imaging. 6nother successful approach to modulate the optical properties is through the design of new bimetallic platinum(II) complexes in which the intramolecular Pt•••Pt distance can be synthetically manipulated, primarily by the nature and bulkiness of the bridging ligands and to a minor extent by the cyclometalating groups.In addition, in bimetallic complexes, the efficiency is usually notably increased, which has been attributed to the enhanced coupling between the T 1 state and higher lying singlet states due to the incorporation of a second Pt center. 7Considerable interest has focused on two main categories, one featuring a butterfly shape and pyrazolate bridging ligands with relatively longer metal−metal separations and the other having a half-lantern shape and bridging thiolates with shorter intermetallic distances (Chart 1).Indeed, very efficient NIR phosphorescent OLED emitters have been reported using single-emissive binuclear half-lantern cycloplatinated complexes with various thiolates (pyridyl-thiolate, oxadiazole-thiolate, or benzo [d]thiazole-2-thiolate) as bridging ligands associated with a very strong 3 MMLCT emission due to relatively short Pt•••Pt distances.5b,8 In the butterfly shape, Pt II complexes bearing pyrazolate bridging groups, the electronic structure mainly depends on the steric bulkiness of the substituent groups of the pyrazolate ligands, which modulates the Pt•••Pt separation not only in the ground state but also in the excited state.When the Pt•••Pt separation is relatively large, the low-lying transitions are due to mixed LC/MLCT excitations located on separated platinum units because, upon excitation, there is a local T 1 minimum with a similar geometry to the S 0 ground state.However, if the Pt•••Pt separation are shorter, upon excitation, easy intersystem crossing (on a subpicosecond time scale) to the 1,3 MMCT state takes place, leading to a T 1 -global minimum in which further contraction of Pt−Pt distance (∼0.2−0.3Å) occurs, due to depopulation of the dσ*, characteristic of the 3 MMLCT excited state. 9In these systems, the photoinduced molecular structural change that takes place upon excitation, strongly depends on the surrounding environment of the molecule and, as a consequence, access to both excited states occurs giving rise to dual emissions depending on the media and the concentration.8a,10 The dynamics and properties of the excited states of these binuclear complexes have been explored by ultrafast spectroscopic techniques (femtosecond absorption spectroscopy) and density functional theory (DFT) calculations in recent years.9a,11 The electronic characteristics of the chromophoric cyclometalating group also play a key role on the final electronic nature of these diplatinum complexes, but its effect has been comparatively less explored. 12n this field, despite the tremendous role that Pt•••Pt nonbonded interactions play on the optical properties of cycloplatinated complexes, much less attention has been devoted to complexes featuring formal covalent Pt−Pt bonds (Chart 1). 13Indeed, in diplatinum cycloplatinated compounds, reports on emissive Pt III −Pt III (d 7 −d 7 ) derivatives are quite rare. 14In these complexes, the lowest lying excited state has usually a remarkable metal center character (dσ M −dσ* M ), being therefore nonemissive.
Heterocycles featuring benzothiazole frameworks have been widely recognized as biologically active compounds 15 and are also important fluorophores used to construct donor/acceptor systems displaying intramolecular charge-transfer properties. 16n this context, over the past few years, we have focused on the design and study of the optical and biological properties of cyclometalated Pt II , Pt ) complexes based on phenylbenzothiazole (pbt) and 2-(4dimethylaminophenyl)benzothiazole (Me 2 N-pbt) as cyclometalated groups and different pyrazole (1) or pyrazolate (2−5) as bridging ligands.Their photophysical properties, supported by theoretical calculations, are presented.
On the other hand, sulfoxides, in particular asymmetric sulfoxides, are nowadays widely used in organic synthesis, fine chemicals, medicine, pesticides, 18 and, recently, have also emerged as efficient ligands in transition-metal catalysis. 19ommon oxidants for sulfide oxidation to sulfoxides (H 2 O 2 , K 2 S 2 O 8 , or m-chloroperbenzoic acid) are usually not ecofriendly or cause serious environmental pollution.In recent years, photocatalytic oxidations have been successfully reported employing O 2 as an environmentally friendly oxidant and different organic photosensitizers, such as Rose Bengal, 20 Rivoflavin, 21 or Bodipy. 22In this area, the employment of metal phosphorescent complexes as photosensitizers is rather less developed; although recently, the efficiency of several Ir III , Ru II and Au I systems have been demonstrated. 23To increase the knowledge on these systems, we sought to explore the utility of complex 3a, which displayed strong oxygen sensitivity on its phosphorescent band, for the photocatalytic oxidation of sulfides using O 2 as a green oxidant.)], due to the anisotropic shield of the aromatic pyrazole ligand.Bis-pyrazole compound 1a displays a broad signal with Pt satellites assigned to the H 3' (8.10 ppm, 3 J Pt−H 20.3 Hz), whereas the derivatives 1b−1c show the corresponding signals of the alkyl substituents on the pyrazole co-ligands in the aliphatic region.Complexes 1a and 1c show two broad singlets at low frequencies due to the N− H protons of the pyrazole groups, whereas complex 1b displays four signals indicating the presence of two conformational isomers in a ca 35:65 molar ratio.We note that in these complexes there are two possible relative orientations of the NH units of the pyrazole ligands in relation to the platinum coordination plane leading to a syn/anti isomerism (Scheme 1b).In complex 1b, the interconversion between both conformers (atropisomers) seems to be slower than the NMR time scale.The presence of both conformers is also apparent in the H 4 ′ ,4 ″ protons (δ 6.40−6.28range, 2H) and methyl resonances (2.47−2.35ppm, 2H) of the nonequivalent 3,5dimethylpyrazole ligands and in the corresponding 13 C{ 1 H} NMR signals (See Figure S2).Finally, 1a−1c complexes display the expected doublet (∼−72.6 ppm, 1 J F−P 706 Hz) and septuplet (∼−145 ppm) in the 31 P{ 1 H} and 19 F{ 1 H} NMR spectra, respectively, corresponding to the PF 6 counteranion.Suitable crystals for complexes 1a and 1b have been obtained by slow diffusion of n-hexane through a solution of the corresponding compound in acetone at low temperatures.In the case of 1a, the X-ray diffraction study (Tables 1, S1 and S2) revealed that this complex crystallized with the PO 2 F 2 anion generated by the partial hydrolysis of the PF 6 − group (1a•  The isomer 3a-anti was also obtained using via (iii).

Inorganic Chemistry
PO 2 F 2 ).Hydrolysis of the PF 6 − anion has been previously observed.17f A view of the molecular structures with the selected bond lengths and angles and a vision of the interaction of the cation and the anions is presented in Figure 1.Both cations showed the expected chelate C^N coordination of the pbt ligand and the cis arrangement of the two R′ 2 -pzH ligands.The bite angle of the pbt ligand [80.4(9)  1).
The bimetallic nature of the complexes was supported by ESI(+) or MALDI(+)-MS analysis, as they show as the parent peak, the corresponding one due to [M + H] + (m/z 945 2a, 1000 2b, 1112 2c, and 1030 3a) (Section S3 and Experimental Section in the Supporting Information).Unfortunately, only complexes 2a and 3a are soluble enough in THF-d 8 to characterize them by 1 H NMR spectra.The anti-isomers (anti-2a and 3a) exhibit the presence of only one set of pbt ligands together with one set of pyrazolate groups.In both complexes, the most deshielded signal corresponds to the H 7 of the pbt, which appear as a doublet (δ 7.89 2a-anti, 7.74 3a), whereas the most shielded is attributed to the H 4' of bridging pyrazolate (δ 6.34 2a-anti, 6.3 3a) (Supporting Information, Section S2).In the anti/syn-2a mixture, the most distinct signal of the type of isomer corresponds to the H 4' of the pyrazolate, which are equivalent for the anti-2a isomer (δ 6.34) and inequivalent in the syn-2a (δ 6.48; 6.32) (Figure S4b).Suitable yellow crystals for X-ray studies of anti-2a and 3a•THF were obtained from slow evaporation of a THF solution.Their molecular structures are depicted in Figures 2 and S11, and the corresponding structural bonding details are provided in Tables 1 and S2.The structures confirm the anti-arrangement of the Pt(R-pbt) units bridged by the two pyrazolate ligands, in coherence with the NMR spectra.Both Pt II complexes display the typical butterfly-like structure with a C 2 symmetry.The intermetallic distances [3.344 2a and 3.1740(4) Å 3a, Table 1] are shorter than the sum of the van der Waals radii of the two Pt (3.5 Å) and comparable to those found in related μ-pyrazolate diplatinum complexes (2.834−3.486Å). 12,26 It has been previously shown that in this type of complexes, both the Pt•••Pt distance and the tilt angle decrease due to the increasing demanding of the substituents of the bridging pyrazolate ligand (3-and 5-positions).26d Also the bulkiness of the cyclometalating group affects to the proximity of the platinum fragments in these type of binuclear complexes. 27urprisingly, despite the presence of bulky NMe 2 groups in 3a, the Pt•••Pt is shorter and the angle between platinum planes is smaller than in 2a (84.23°for 2a and 73.50°for 3a), indicating stronger interactions between the platinum fragments, a fact that might be attributed to the coplanarity between the NMe 2 and the bt unit.On the basis of previous results, it is suggested that the Pt•••Pt separation could likely be shorter in complex 2b (R' = Me) and, particularly, in 2c featuring R' = Pr i groups, what agrees with their intense orange colors.The Pt−C and Pt−N bond distances around the Pt centers are similar to those found in related complexes.In particular, the Pt−N pz distance trans to the C R-pbt [2.099(2) 2a and 2.103(5) Å 3a] is longer than that of trans to the N of the R-pbt group [2.000(2) 2a and 2.004(5) Å 3a] (Table 1), in coherence with the high trans influence of the metalated carbon.The extended structure of both compounds shows weak π•••π interactions between cyclometalated groups of different molecules giving rise to dimers (3.585−3.641Å) supported by secondary intermolecular sulfur These dimers additionally stack giving rise to a 1D infinite We observed that complexes [Pt(R-pbt)(μ-pz)] 2 (R = H 2a, Me 2 N 3a) are stable in CHCl 3 solution in the dark, but in the presence of sunlight they evolve slowly (∼24 h) to form the metal−metal-bonded Pt III −Pt III complexes [Pt(R-pbt)(μ-pz)-Cl] 2 (4a, 5a), which precipitate in the mixture in a ca yield of 60%.This type of two center-two electron oxidation reactions have been previously studied and depend on many factors involving either a radical-like mechanism (with the O 2 acting as a radical R* trap) and/or a thermally or photochemically activated processes.26h, 28 Recent studies on pyrazolate-and thiolate-bridged diplatinum complexes with very short Pt•••Pt distances indicate that the 1 MMLCT excited states easily triggers the photooxidation of these complexes with CHCl 3 .10b As expected, the complexes were also obtained by reacting 2a and 3a with iodobenzenedichloride (PhICl 2 ) in CH 2 Cl 2 at 0 °C, being precipitated and separated of the mixture in a 59−73% yield (Scheme 3).Complexes 4a and 5a have been characterized by mass spectrometry and NMR spectroscopy (Experimental Section and Sections S2 and S3 in the Supporting Information) and their structures confirmed by X-ray.The formation of the oxidized species is evident by the presence of peaks due to [M-Cl] + (m/z 979 4a, 1065 5a) as parent peaks in their mass spectra.
The 1 H NMR spectra of 4a and 5a display only one set of signals for the R-pbt groups and for the pyrazolate ligands, indicating an anti-arrangement of the cyclometalated group.The ortho protons to the cyclometalated ligand, H 11 , are notably shielded and the three-bond platinum-coupling constant ( 3 J Pt−H 31.1 4a, 35.2 Hz 5a) is smaller than in complex 1a (43.3 Hz), in agreement with the increased oxidation state.The molecular structures of complexes 4a and 5a•0.5CH 2 Cl 2 were determined by single-crystal X-ray diffraction (Table 1 and Figures 3 and  S12).The Pt III −Pt III complexes retain the boat-like structure of the Pt 2 N 4 core and the anti-arrangement of the platinum(C^N) fragments.The interplanar angles between the platinum coordination planes are notably reduced with respect to those observed in Pt II −Pt II precursors (35.68°4a and 34.30°5a vs 84.23°2a and 73.50°3a) due to the formation of the Pt−Pt bond.26h As expected, the formation of a Pt−Pt formal bond produces a shortening in the Pt−Pt distance with respect to complexes 2a and 3a [2.5897(2) 4a 2.5776(3) Å 5a vs 3.344 2a, 3.1740(4) Å 3a, Table 1].The molecular packing of these complexes shows extended π•••π interactions through the R-pbt groups with distances of 3.458−3.544Å in 4a and 3.416−3.538Å for 5a, forming 1D chains (Figures 3 and S12).
Optical Properties.Absorption Properties and TD-DFT Calculations.The UV−vis spectra of bis(pyrazole) complexes 1a−1c in THF (5 × 10 −5 M) solution and in the solid state (Table S3, Figures 4 and S13a) are rather similar, indicating a negligible influence of the pyrazole groups.In solution, they show one intense absorption at ca. 260 nm, attributed to spinallowed π−π* intraligand ( 1 IL, pbt) transitions.They exhibit an additional band in the 300−370 nm range and a less intense feature in the low-energy region at 385−420 nm (ε ∼ 10 3 L• mol −1 •cm −1 ).On the basis of theoretical calculations carried out in complex 1a (Figure 4 and Section S5, Supporting Information), which indicate that the calculated low energy transition (S 1 382 nm) is mainly contributed by the HOMO (80% pbt, 20% Pt) to LUMO (93% pbt, 6% Pt) excitation, the low-energy feature is mainly attributed to an intraligand 1 IL (pbt) with some 1 MLCT.The band in the range of 300−370 nm has also a mixed 1 MLCT/ 1 IL character.The Hpz orbitals contribute from the LUMO+1 (63%) and they are involved in the S 6 /S 8 transitions with very low oscillator strength (Table S4 and Figure S15).
As noted, the binuclear Pt II −Pt II complexes 2b and 2c are insoluble in common solvents.Therefore, only the absorption spectra of the pyrazolate bridging complexes 2a and 3a could be recorded in THF solution (Figure 5).In both compounds, the band at λ < 350 nm are ascribed to π−π* intraligand ( 1 IL, pbt) and ligand-to-ligand (R 2 pz to pbt) transitions.In 2a, the bands between 350 and 410 nm are ascribed to mixed 1 IL/ 1 MLCT and the low energy feature at ca. 440 nm to 1 MMLCT transitions, as supported by calculations (see Table S4).In the Me 2 N-pbt 3a, the bands are more intense indicating a stronger intraligand charge-transfer contribution (NMe 2 to bt) and are slightly redshifted (3a 430, 450 nm ε ∼ 20 × 10 3 vs 2a 384, 440 nm ε 4.78 × 10 3 ).As seen in Figure 5, which shows the frontier orbitals for Inorganic Chemistry both complexes, in both, the target LUMO and L+1 spread on the cyclometalated ligands.However, whereas in 2a, the highest orbital HOMO has a σ* (5d z 2 −5d z 2 ) character and is located on the two platinum atoms; in 3a, a similar orbital is the H-2.In 3a, the HOMO and HOMO−1 are destabilized in relation to 2a and are located on the Me 2 N-pbt.For 2a, the calculated S 1 (452 mn) and S 2 (439 nm) have 1 MMLCT characters, whereas for 3a the most intense calculated absorptions are S 2 (423 mn) and S 3 (415 nm) and have mixed 1 ILCT/ 1 MMLCT.It is interesting to note that in both complexes the calculated transitions with remarkable metal platinum contribution and mixed 1 MLCT/ 1 ILCT character appear at higher energy (S 4 397 nm 2a; S 7 351 nm 3a).The solid-state absorption spectra of 2a−2c have been also recorded.The main lowest-energy band ranges from 438 (2a), 455 (2b) to 503 nm (2c), with extending tails, in agreement with their color (Table S3 and Figures S13, S14), what are ascribed to mixed 1 IL/ 1 MMLCT, with higher contribution of this latter on going from 2a to 2c, likely due to a lower Pt−Pt distance by increasing the steric bulk of the substituents.Low intense features in the tails (>480 nm 2a, 2b or  540 nm, 2c) are observed, which are probably of spin-forbidden nature.
The absorption spectra of Pt III −Pt III 4a and 5a are included in Figure 6.The most significant feature in relation to complexes 2a and 3a is the notable hypsochromic shift in the low energy region.Thus, 4a displays a moderately intense absorption low energy band at 348 nm with a shoulder at 379 nm and 5a a band at 402 with a shoulder at 444 nm, clearly blue-shifted in relation to the corresponding Pt II −Pt II (440 2a, 430 nm 3a), reflecting to the oxidation of the platinum centers and the notable change in the frontier orbitals.DFT and time-dependent DFT (TD-DFT) calculations were performed with the Gaussian 16 program package to explore the orbital frontiers and the nature of the excited states and transitions (Tables S4−S7 and Figure S16).Figure 6 includes a selection of the orbitals and excitations.In both complexes, the LUMO is essentially identical located on  the ClPtPtCl axis and formed by the antisymmetrical σ* combination of the 5d z 2 of the two platinum centers and the p z orbitals of the chloride atoms (58% Pt−Pt and 26% Cl−Cl).Therefore, population of this orbital should cause strong distorted excited states with elongation of the Pt−Pt separation and of the Pt−Cl bonds.The symmetrical combination σ of the 5d z 2 is mainly located on the HOMO−2, but this orbital has also a notable contribution of the phenylbenzothiazolate groups.For complex 5a, the HOMO and H-1 are primarily located on the Me 2 N-pbt groups, while in the pbt complex, 4a have also small platinum and Cl contributions (see Table S7).The calculated low lying S 1 −S 3 (431 to 489 nm 4a and 748−489 nm 5a) transitions have very low oscillator strength and complex configuration (mixed LMMCT/LXCT/L'XCT 4a and LMMCT/LXCT/ILCT 5a) (L = R-pbt, X = Cl, L' = pz).The most intense low energy-calculated band is S 4 ascribed to H-4, H-2 to LUMO in 4a and to a more complex configuration in 5a (H-2, H-5, H-6, and H-10 to LUMO) having a mainly L'MMCT/L'XCT nature in 4a and LMMCT/L'MCT/XC/ MC in 5a.The solid-state spectra also reflect the oxidation of the Pt centers exhibiting blue-shifted low-energy features in relation to the Pt II −Pt II precursors (see Table S3, Figure S14).Emission Properties and DFT Calculations.The emission spectra of complexes 1 and 2a−5a were registered in THF fluid solution (5 × 10 −4 M for 1 and 2a−3a and 5 × 10 −5 M for 4a), THF glasses at 77 K, polystyrene (PS) films at 10% wt, and in the solid state for all complexes at room and low temperatures (Tables 2 and S8, Figures 7−10 and S17−S19).Mononuclear complexes 1a−1c show in the solid state, THF solution and doped films at 298 K a similar long-lived vibronically structured emission band (λ max ∼ 540 nm), slight blue-shifted at low temperatures (λ max ∼ 530 nm), attributed to a predominant intraligand transition ( 3 IL) with some metal-to-ligand chargetransfer ( 3 MLCT) characters.This emission is comparable to those reported for related neutral heteroleptic monomers [Pt(pbt)(L^X)] (L^X = O^O, N^O, P^O), 4d,17e,29 and the assignment is further supported by optimization of the triplet excited (T 1 ) state for complex 1a + .As illustrated in Figure 7, the spin density surface at the optimized T 1 state is centered on the pbt ligand with some contribution of the platinum (Pt ∼ 0.13).The adiabatic calculated emission wavelength (638 nm) exhibits the expected overestimated value in relation to the experimental data (∼540 nm, THF, 298 K).The calculated quantum yields are moderate in solution (10−21%, deoxygenated conditions, Table 2) but relatively low in rigid media (solid and PS, 1−8% Table S8), indicating aggregation-caused quenching characteristics (ACQs). 30The relatively strong π•••π and S••π interactions between the pbt ligands in these rigid media, as found in their Xray structures, could provide easy deactivation pathways and may account for the reduced efficiencies.Notwithstanding, it is worth noting that reports on luminescent mononuclear bispyrazole Pt II complexes are rare. 31A comparison of the radiative and nonradiative rate constants (k r and k nr ) reveals that complex 1a presents the highest k nr value in relation to 1b and 1c (k nr 2 × 10 5 1a vs 1.4 × 10 5 1b, and 1.5 × 10 5 1c) and the lowest k r (2.2 × 10 4 1a vs 3.5 × 10 4 1b and 4 × 10 4 1c), resulting in a less efficient phosphor (φ 10% 1a vs 20% 1b and 21% 1c).This could be associated with a less steric hindrance of the pyrazole ligand in 1a, which provides less rigidity.
The bimetallic complex 2a displays in all media a wellresolved vibronic emission profile with λ max (∼540−550 nm) (Figure 8), close to that observed for complexes 1, being therefore ascribed to a local mixed 3 IL/ 3 MLCT excited states.This behavior could be related to the relatively long Pt•••Pt of 3.344 Å distance found for this complex, which makes it difficult to reach the σ 2 -σ* 1 configuration characteristic of the 3 MMLCT excited state.10a,11b,26k The optimized S 0 structure for complex 2a shows a Pt−Pt separation of 3.2069 Å, comparable to that found in the crystal structure, and similar separation is found in the optimized T 1 excited state (see Table S5), indicating the absence of short intramolecular platinum−platinum bonding interaction typical of the 3 MMLCT excited state in T 1 .To confirm the nature of the emission, the lowest T 1 −T 3 vertical excitations at the S 0 geometry and the corresponding optimized T 1 −T 3 were calculated (Tables S4, S9, S10 and Figure 8b).The two lowest vertical triplet excitations T 1,2 , which are close in energy (2.4 and 2.5 eV), and their corresponding optimized T 1,2 states (676 nm) are located on one of the Pt(pbt) fragments having mixed 3 IL/ 3 MLCT nature.As illustrated in Figure 8b, the following excitation, T 3, has a 3 MMLCT character but is located at 0.3 eV above T 1 .In 2a, the metal contribution in T 1 increases in relation to the mononuclear complex 1a (0.2125 2a vs 0.1303 1a, Table S9) pointing to a higher metal-to-ligand chargetransfer ( 3 MLCT) contribution in 2a, in correlation with the lower lifetime recorded in THF solution for this bimetallic complex in comparison to the 1a one (4.5, 1a vs 0.9 μs, 2a).The recorded quantum yield for 2a in deoxygenated THF solution (1%) is lower than for 1a (10%), but relatively similar in solid and PS (Table S8).This decrease in the quantum efficiency is attributable to an appreciable increase in k nr (1.1 × 10 6 2a vs 2.0 × 10 5 1a), while there is a decrease in k r (1.1 × 10 4 2a vs 2.2 × 10 4 1a).
For complexes 2b and 2c, only emission properties in the solid state were recorded (Figure 8c,d).Dimethylpyrazolate-bridge derivative (2b) shows, at room temperature and at 77 K, a similar band to that obtained for complex 2a (∼550 nm) indicating emission from an 3 IL/ 3 MLCT excited state.For complex 2c, with a bulkier substituent on the bridge group (R = i Pr), the emission is broader at 298 K peaking at 560 nm and well stylized and red-shifted to 600 nm at 77 K (Figure 8d).This fact suggests the contribution of the low lying 3 MMLCT excited state, which becomes predominant by decreasing the temperature.
Complex 3a, comprising the donor−acceptor Me 2 N-pbt groups, displayed a significantly different behavior from that of the related complex 2a (Table 2).It exhibits, upon excitation on the low energy band at 400−450 nm, and in a carefully deoxygenated THF solution at 298 K, in addition to a strong long-lived low energy band (LE) in the yellow-orange region (570 nm), a minor high energy feature (HE) in the blue-green Inorganic Chemistry region (500 nm) (see Figure 9a, orange line).The high energy feature is short-lived (1.5 ns) and displays characteristic mirror band-shaped with the longest wavelength absorption band, being therefore ascribed to S 1 → S 0 fluorescence having metalperturbed intraligand charge-transfer 1 ILCT (Me 2 Nph-to-bt) characters, whereas the LE-structured phosphorescent band is ascribed, according to calculations, to 3 ILCT.In this complex 3a, the two first triplet excitations T 1,2 at the S 0 geometry are essentially isoenergetic (556 nm) and exhibit an ILCT character (Figure 10c).The T 3 , with a MMLCT character, lies more separated from T 1 (0.6 eV) than in 2a (0.3 eV).The spin density distribution on the optimized T 1 is mainly located on one of the cyclometalated groups with a lower metallic contribution in relation to the pbt-derivative 2a (0.0809 3a vs 0.2125 2a), supporting primarily 3 ILCT nature for the phosphorescent emission (Table S9).As was expected, the ratio F/P clearly increases in oxygenated solution due to a partial quenching of the low energy phosphorescent band (Figure 9a, blue line).The determined phosphorescence quantum yield in degassed solution is 17% and is reduced to 2% in air equilibrated solution.The excitation spectra of both bands correlate with the absorption spectrum in the low energy region, indicating that both emissions came from the same complex.However, the excitation spectra are not exactly identical in the region around  365−400 nm, where there is a notable MLCT contribution.This suggests that, while the fluorescent HE band mainly proceeds of excitation of the 1 ILCT, the phosphorescent LE band is also notably populated from high-energy S n excited states.Interestingly, we also observe that the emission depends on the excitation wavelength.Thus, upon excitation at 365 nm, a dual emission is also observed formed by a short-lived blueshifted fluorescence band (1.6 ns) located at 405 nm and the structured low energy phosphorescent at 570 nm, which is extremely air sensitive being nearly absent in air-equilibrated THF solution (decreases from 23% in degassed solution to less of 1% in air equilibrated).The new fluorescence band is related to an excitation manifold located at 350 nm, whereas the excitation spectrum when detected the LE band is identical to that observed for the LE band upon exciting in the low energy region (400−450 nm) (Figure 9b).Our calculations suggest that the excitation S 7 , at 351.3 nm, has mixed 1 MLCT/ 1 LC nature with a remarkable platinum contribution and minor contribution of the NMe 2 (Figure 10b).In agreement with this, the high quantum efficiency in 3a upon excitation to 365 nm (ϕ = 23%) is mainly attributable to its higher k r (2.6 × 10 4 ) and lesser k nr (8.9 × 10 4 ) in relation to those obtained upon excitation to 400−450 nm (Table 2), probably related to the higher metal contribution in the LE emission using the highenergy excitation wavelengths.This wavelength dependence suggests the occurrence of hyper-intersystem crossing (HISC) from S 7 ( 1 MLCT/ 1 LC) to T 1 , i.e., relaxation from S 7 to T 1 clearly competes with internal conversion (IC) to S 1 .It seems that the energy transfer from relaxed ( 1 MLCT/ 1 LC)* to 1 ILCT* is nonefficient.This relatively rare behavior has been previously observed in some platinum complexes, being related to the presence of relaxed S 1 states having strong ππ* characters with an essentially null metal contribution.17d,32 At 77 K, the fluorescence is lost and only the phosphorescence emission band at 565 nm is developed regardless of the wavelength used in the excitation.
Interestingly, we observed that in aerated THF solution, upon prolonged photoexcitation at 365 nm, a continuous enhance-ment of the phosphorescent band is observed, rising its maximum intensity in ca 27 min (Figure 10b).The process is also visible with a hand UV−vis lamp in which the initial violet emission, attributed to 1 MLCT/ 1 LC fluorescence, was gradually changing to a final orange enhanced emission (Figure 10a).The intense orange emission was switched off by simply shaking the solution allowing its oxygenation.A similar process, which is reversible and can be repeated several times, was also observed in DMSO but does not take place in other solvents, such as acetonitrile, toluene, or MeOH.This relatively rare behavior has been previously observed for us 17d and other groups, 33 being explained by the occurrence of a local sensitization caused by energy transfer from the low energy triplet to 3 O 2 producing singlet 1 O 2 able to selectively react with the solvent (THF 34 or DMSO 17d ), thus creating a free oxygen microenvironment that switch-on the phosphorescence ( 3 ILCT).The strong sensibility of the phosphorescent emission in this complex (see Table 2) encourages us to determine its efficiency as an 1 O 2 sensitizer.The singlet oxygen generation of complex 3a was examined in acetonitrile solution (5 × 10 −4 M) on the infrared region detecting the characteristic emission profile of 1 O 2 at λ em ∼ 1274 nm (Figure S21) upon excitation at both 365 and 400 nm, respectively.Phenalenone (PN), a universal reference compound which can be used in various solvents, 35 has been employed.The measured quantum yield (ϕ Δ ) of 1 O 2 was notably higher upon exciting at 365 nm than upon exciting at 400 nm (37% vs 11%), in agreement with the higher 3 ILCT phosphorescence efficiency (23% vs 17%, Table 2).These values indicate that this complex can be used as a photosensitizer.
In the solid state, this complex (3a) displays a broader emission red-shifted at low temperatures (Figure S19), indicating some additional 3 MMLCT contribution.
Because the initial structural report of a diplatinum d 7 −d 7 complex, K 2 [Pt 2 (SO 4 ) 4 (H 2 O) 2 ], 36 different types of high-valent diplatinum d 7 −d 7 , either with bridging and unbridged ligands, have been reported. 37These complexes have attracted a great interest in many efficient catalyst processes, such as the oxidation of unsaturated organic molecules, facilitated by the strong electron-withdrawing ability of the unusually high oxidation state of the Pt III atom.37a Initial suggestions 38 and recent contributions 13,39 support that these complexes can be sometimes key intermediate in the oxidation pathway from Pt II to Pt IV .Indeed, their rich reactivity indicates that the electron distribution along the Pt−Pt bond can be viewed as a resonance structure between Pt III −Pt III and Pt II −Pt IV .37e,40 Despite their rich chemistry, and also the well-known emissive properties of either Pt II and Pt IV complexes, reports on emissive d 7 −d 7 derivatives are quite rare, mainly due to the fact that in these complexes the target orbital and therefore, the lowest-lying excited state possess metal−metal antibonding (dσ* M-M ) character, 28 being therefore short life and nonemissive.To the best of our knowledge, only three types of luminescent binuclear Pt III complexes have been reported: (i) of the type [Pt 2 (μpop) 4 X 2 ] 4− (pop = P,P-pyrophosphite, P 2 O 5 H 2 2− ; X = Cl, Br, SCN) or [Pt 2 (μ-pop) 4 X 2 ] 2− (X = py); 41 (ii) type [Pt 2 (μ-C 6 H 3 -5R-2-AsPh 2 ) 4 X 2 ] (R = methyl o isopropyl, X = Cl, Br, I) 14a and (iii) [Pt(C^N)(μ-pxdt)] 2 (pxdt = oxadiazole-thiol).In these latter, donor−acceptor bridging ligands have been employed giving rise to emission from an excited state having ligand-tometal−metal charge transfer-(LMMCT) character.14b The Pt III −Pt III complexes (4a and 5a) are weakly emissive, showing in PS rigid media and in glassy THF solution (only 4a), a structured low-efficient (2% 4a and <1% 5a in PS) emission profile associated with a 3 IL character centered on the R-pbt group (550 4a and 566 nm 5a) (Figure 11).The lowest TD-DFT (T 1,2 for 4a and T 1−3 for 5a) vertical excitations at the S 0 geometry (Table S4) are of LMMCT/LXCT in nature and are expected to be not emissive, in accordance with the lack of emission in fluid solution.The weak emission observed in rigid media is tentatively associated with close higher excited states (T 3 for 4a and T 4 for 5a) having a mainly 3 ILCT character with a minor 3 MLCT/ 3 XLCT additional contribution for 4 (see Table S4).The calculated values (487.5 nm 4a and 603 nm 5a) agree with the experimental red shift observed for 5a relative to 4a attributed to the incorporation of the donor NMe 2 group, which increases the energy of the HOMO decreasing the gap of the transition.
Electrochemical Properties.The electrochemical properties of complexes 2a−5a were investigated using cyclic voltammetry in anhydrous CH 2 Cl 2 with (NBu 4 )PF 6 as a supporting electrolyte in the dark.
The potentials and HOMO/LUMO energy estimations are listed in Table 3 and voltammograms covering the anodic (2a, 3a) and cathodic (2a−5a) regions are depicted in Figure S22.Cyclometalated diplatinum complexes with butterfly or halflantern shape exhibit mainly a two-electron oxidation process, assignable to the oxidation of the divalent species Pt 2 (II) to trivalent species Pt 2 (III); 8b,d,12,42 although in some cases, two one-electron redox waves corresponding to the Pt 2 (III)/Pt 2 (II) couple were observed. 43The Pt II −Pt II complexes 2a and 3a show in the anionic window two bad resolved quasi-reversible or irreversible process with E ox 0.63, 0.96 V 2a, 0.73, and 1.07 V 3a (vs Ag/AgCl), probably due to two steps of one-electron oxidation from Pt 2 (II,II) to Pt 2 (III,II) and to Pt 2 (III,III).The irreversibility of the oxidation processes is caused by the nucleophilic reactions of the coordinating solvent to the electrogenerated Pt III species. 12The Pt III −Pt III complexes 4a and 5a do not exhibit significant redox peaks by scanning the potential in the anodic direction.
Complexes constructed with the NMe 2 -pbt cyclometalating ligand show an irreversible reduction wave with shape and potential values that are similar (E red −1.10 V Pt 2 II 3a, −1.07 V Pt 2 III 5a).However, the pbt complexes displays different reduction behavior.Thus, whereas the Pt 2 II complex 2a shows an irreversible wave at −1.01 V, two reduction waves at −0.69 and −1.21 V were resolved for 4a (Pt 2

III
). HOMO and LUMO  Photocatalytic Studies.In recent years, visible light photocatalysis has received a great attention allowing to furnish new molecules and structural motifs with lower energy consuming when compared with reactions under thermal or ultraviolet (UV). 44In this area, luminescent cyclometalated transition metals (Ru II , Ir III and Pt II ) are among the most frequently employed sensitizers for energy transfer and photoredox catalysis. 45In particular, some of these complexes have been previously described as efficient photosensitizers for the generation of reactive oxygen species ( 1 O 2 , O 2 •− ), being successfully employed as photocatalysts for the photooxidation of different organic molecules. 46Among the various oxidation reactions, the photo generation of sulfoxides from sulfides using oxygen as oxidant is of great interest, 21,23a,c,47 mainly due to its relevance in the synthesis of biologically active compounds used in the pharmaceutical industry and also in organic synthesis. 18e recently reported the ability of dimethylphenylbenzothiazole platinum complexes to generate, under photoexcitation, sensitized singlet oxygen ( 1 O 2 ) able to induce oxidation of DMSO to DMSO 2 .17d In this line, the good ability of complex 3a to photo sensitize singlet oxygen encouraged us to estimate its photocatalytic activity.In particular, we investigated the induced photooxidation ability of p-bromothioanisole (Scheme 4) under visible light (blue light; λ = 460 nm) in the presence of 3a and oxygen as a model for heterogeneous catalysis. 48The evolution of the catalysis was monitored by NMR spectroscopy.Photosensitizer 3a showed a good photostability under irradiation of blue light in suspension and solid state (measured by UV−vis spectroscopy from 0 to 50 h, Figure S23).This reaction was evaluated in different conditions and molar ratio of catalyst and substrate (Table 4, entries 1 and 2), with 1 and 5 mol % of the metal complex.In both cases, the reaction occurred with similar results allowing >95% conversion after 50 h of reaction.However, with 5% the reaction shows slight greater efficiencies at shorter times (3−4 h of reaction, Table S11).As illustration, after 15 h of reaction with 1% showed a 45% conversion, while with 5% a 55%.These results allow us to conclude that the increase of the amount of catalyst does not produce remarkable changes in the efficiencies.
When the reaction was carried out with double amount of catalyst and substrate (Table 4, entry 3), the 95% of conversion was reached at 36 h, revealing that the increment in the concentration of the photocatalytic reactions improves the efficiency.Under hypoxic conditions, the conversion was considerably reduced because of the impossibility to produce ROS in the absence of oxygen (Table 4, entry 4).In the absence of light (Table 4, entry 5), no conversion was observed and, similarly, without a catalyst (3a) in the presence of air the reaction does not take place (Table 4, entry 6).
Generally, two mechanisms involving two main ROS intermediates such as 1 O 2 and O 2 •− have been proposed for the photocatalytic oxidation of sulfides: 49 (a) The photooxygenation promoted by 1 O 2 generated by a photosensitizer and (b) a photosensitized electron-transfer (ET) oxidation using 3 O 2 through O 2 * − as an intermediate.In both mechanisms, a similar zwitterionic persulfide intermediate (R 1 R 2 S + O−O − ) is proposed, which reacts with a second molecule of R 1 R 2 S leading to the formation of two molecular sulfoxides.Discrimination between both mechanisms is not an easy task. 21To get insights into the catalytic mechanism, several control experiments were carried out.Thus, the photocatalytic reaction was evaluated out in the presence of 1,4diazabicyclo[2.2.2]octane (DABCO), a well-known singlet oxygen quencher, to evaluate the potential production of these species.As can be observed in the Table 2-entry 7, in the presence of DABCO (3 equiv), reagents (sulfide, 3a), and light, the reaction is notably reduced (10%) and only traces of oxidized product were obtained, pointing to a key role of complex 3a as an oxygen sensitizer.On the other hand, the addition of a superoxide radical (O 2 •− ) quencher like 1,4benzoquinone (BQ, 3 equiv) to the photooxidation process (Table 4, entry 8) also results in a remarkable decrease of the reaction yield to 23%, revealing a key role of these reactive oxygen species too.These observations suggest that both 1 O 2 and superoxide radicals play an important role in this photocatalytic reaction.

■ CONCLUSIONS
In summary, we have prepared novel series of mononuclear (1a−c), bimetallic (Pt II −Pt II ) (2a−c, 3a), and (Pt III −Pt III ) (4a, 5a) complexes incorporating phenylbenzothiazole (pbt) and 2-(4-dimethylaminophenyl)benzothiazole (Me 2 N-pbt) as cyclometalating chromophore groups and pyrazole (1) or pyrazolate bridging ligands (2−5).Experimental data and computational studies reveal the negligible influence of the pyrazole or  Inorganic Chemistry pyrazolate bridging ligand on the optical properties of complexes 1a−c and 2a,b, which exhibit typical low-lying IL/MLCT electronic transitions.Only complex 2c, incorporating the bulky 3,5-i Pr 2 pz bridging groups exhibits in the solid state an emission with some contribution of the 3 MMLCT excited state, which is clearly predominant at low temperatures (560 nm at 298 K; 600 nm 77 K). 3a incorporating the donor−acceptor Me 2 N-pbt ligand displays unusual dual, Fluorescence ( 1 ILCT or 1 MLCT/ 1 LC) and phosphorescence ( 3 ILCT) emissions depending on the excitation wavelength.The efficiency of the population of the triplet manifold increases upon photoexcitation of excited states having a higher metal d contribution.
The phosphorescence can be reversibly photoinduced in oxygenated THF and DMSO solutions upon continuous excitation (365 nm, ∼ 15 min) and quenched by shaking.The complex also photosensitizes 1 O 2 , with a higher quantum yield at λ ex of 365 nm than at 400 nm (37 vs 11%).Computed results for the low-lying T 1 −T 3 excited states in complexes 2a and 3a indicate that T 1,2 have a mixed 3 IL/ 3 MLCT nature in 2a and mainly 3 ILCT character in 3a.In both complexes, the T 3 has a 3 MMLCT character and lies more separated from T 1 (0.6 eV) in 3a than in 2a (0.3 eV).The diplatinum complexes 4a and 5a increase the small number of luminescent d 7 −d 7 compounds reported, with a weak emission, in rigid media, ascribed to 3 ILCT.Finally, complex 3a, which demonstrates the ability to photosensitize singlet oxygen, has been further applied for the photooxidation of p-bromothioanisol under visible light (460 nm).Control of this reaction suggests that both, 1 O 2 and superoxide radicals, play an important role in this photocatalytic reaction.
Characterization of complexes (NMR spectra, crystal data), photophysical properties and computational details, and photocatalytic studies (PDF)

Figure 2 .
Figure 2. Molecular structure and crystal packing of complex 2a.

Figure 4 .
Figure 4. UV−vis absorption spectra of mononuclear complexes 1 and schematic representation of selected orbitals frontiers and transitions for 1a.

Figure 5 .
Figure 5. UV−vis absorption spectra of mononuclear complexes 2a and 3a in THF (5 × 10 −5 M) and an schematic representation of their frontier orbitals and selected excitations.

Figure 6 .
Figure 6.UV−vis absorption spectra in THF solution (5 × 10 −5 M) of 4a and 5a and an schematic representation of their frontier orbitals and selected excitations.

Figure 8 .
Figure 8.(a) Emission spectra (λ ex 400−450 nm) of complex 2a in THF solution at 298 and 77 K. (b) Relative energies and character of the vertical triplet excitations (T 1 −T 3 ) for complex 2a.Emission spectra of 2a−c in solid state (c) at 298 (d) at 77 K.

Figure 10 .
Figure 10.(a) Photographs showing the switch-on and enhancement of the phosphorescent emission of 3a in an aerated THF solution upon excitation at 365 nm.(b) Emission spectra of 3a in THF 5 × 10 −4 M solution in the presence of O 2 with different irradiation times.(c) Relative energy and character of the more intense vertical singlet and triplet excitations at the S 0 geometry of 3a.

Table 3 .
Electrochemical Data a and HOMO/LUMO Energy Estimations for Complexes 2a−5a Estimated HOMO/LUMO energy by electrochemistry data [E HOMO/LUMO = −(E ox/red +4.8 − E Fc/Fc+ )]. e Estimated HOMO/LUMO energy by DFT calculations.energylevels were estimated from these CV data by using the relationship E HOMO/LUMO = −(E ox/red + 4.8 − E Fc/Fc+ ), where E Fc/Fc+ (0.45 V) is the potential of ferrocene vs Ag/AgCl and 4.8 eV is the energy level of ferrocene to the vacuum energy level.The calculated LUMO energies are between −3.13 and −3.35 V, being similar for the NMe 2 -pbt complexes (−3.24 3a, −3.27 V 5a), what is an accordance with a LUMO mainly located in the cyclometalated ligand.Notwithstanding, the estimated HOMO and LUMO energies do not correlate well with those obtained by DFT calculations.