A Platinum–Dithiolene Monoanionic Salt Exhibiting Multiproperties

May 10, 2016 - Synopsis. The platinum salt C[PtL2], where C = [(R)-Ph(Me)HC*-NMe3]+ and L = [4′, 5′: 5, 6][1, 4]dithiino[2,3-b]quinoxaline-1′,3â...
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A Platinum−Dithiolene Monoanionic Salt Exhibiting Multiproperties, Including Room-Temperature Proton-Dependent Solution Luminescence Salahuddin Attar,† Davide Espa,† Flavia Artizzu,† M. Laura Mercuri,† Angela Serpe,† Elisa Sessini,† Giorgio Concas,‡ Francesco Congiu,‡ Luciano Marchiò,*,§ and Paola Deplano*,†,‡ †

Dipartimento di Scienze Chimiche e Geologiche and INSTM and ‡Dipartimento di Fisica, Università di Cagliari, S.S. 554-Bivio per Sestu, I09042 Monserrato-Cagliari, Italy § Dipartimento di Chimica, Università di Parma, Parco Area delle Scienze 17A, I43124 Parma, Italy S Supporting Information *

ABSTRACT: The platinum salt C[PtL2], where C = [(R)Ph(Me)HC*-NMe3]+ and [PtL2]− = radical monoanion based on [4′, 5′: 5, 6][1, 4]dithiino[2,3-b]quinoxaline-1′,3′dithiolato, shows a variety of properties both in solution and in the solid state thanks to the electronic and/or structural features of the ligand. The complex crystallizes in the chiral space group P1 due to the presence of the enantiopure cation (R)-Ph(Me)HC*-NMe3+, and it shows paramagnetic behavior relatable to the [PtL2]− radical monoanion. This anionic complex is redox active and shows a strong near-infrared absorbance peak at 1085 nm tunable with the oxidation state of the complex. This complex exhibits a proton-dependent emission at 572 nm in solution at room temperature. The excitation band corresponds to the HOMO−1 (π-orbitals of the S2C2S2 system) → LUMO (π-orbitals of the quinoxaline and benzene-like moieties) transition suggesting that emission is mainly ligand centered in character. The luminescent properties are highly unusual, since the emission falls well above the energy of the lowest energy absorption (anti-Kasha behavior). Joint experimental and density functional theory (DFT) and time-dependent DFT studies are discussed to provide a satisfactory structure/property relationship.

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INTRODUCTION Metal dithiolene complexes form a well-known class of compounds whose peculiar electronic structures determine their conducting,1 magnetic,2 and optical3 properties. In agreement with a suggestion by Ward and McCleverty,4 the term dithiolene is employed irrespective of the real form of the ligand in a particular complex, in which the ligand is capable to bind a variety of metals as ene-1,2-dithiolate dianion, mixedvalence thioketone-radical thiolate monoanion, or neutral dithioketone. These different ligand forms can be discriminated through structural data concerning the C−C and C−S bond distances.5 It is useful to recall that the most accessible redox state for square-planar d8 metal−dithiolenes is related to the fairly isolated frontier π-molecular orbitals (MOs), which are the out-of-phase (b2g) and in-phase (b1u) combinations, respectively, of a C2S2 orbital in antibonding interactions within one ligand, as shown in Scheme 1 for an [M(S2C2R2)2]n series spanning from n = −2 to n = +2. The electronic effects on the frontier orbitals of various substituents at the dithiolene core have been quantified, and in general it is found that π-acceptor substituents lower the energy of both these orbitals, while π-donor substituents raise their energy. Thus, tunability of metals and R substituents allows tuning the © XXXX American Chemical Society

Scheme 1

nature of the most accessible state and related properties of d8 metal−R2C2S2 complexes from the dianionic state with strong electron-withdrawing ligands to the dicationic state with strong electron-donating ligands.3c Different functional properties can be achieved depending on the redox state of complexes: (1) the neutral diamagnetic complexes, where only the low-lying orbital is populated (b1u, see Scheme 1), behave as a near-infrared (NIR) dye due to an Received: October 30, 2015

A

DOI: 10.1021/acs.inorgchem.5b02491 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(quinoxdt)2]·DMF. Anal. Calcd for C34H33N6OPtS8 (993.23): C 41.11, H 3.35, N 8.46, S 25.83; found: C 41.3, H 3.5, N 8.5, S 25.2%. UV−vis (in DMF solution): λ/nm (ε/M−1 cm−1) 354 (2.04 × 104); 455 (7.01 × 103); 635sh (0.66 × 103); 1085 (1.75 × 104). FT-IR (KBr): νmax/cm−1 3056 (vw); 3034 (vw); 1554 (w); 1479 (w); 1455 (m); 1388 (vs); 1332 (m); 1255 (vs); 1177 (vs); 1112 (vs); 954 (w); 866 (w); 841 (w); 759 (m); 708 (m); 598 (m); 465 (vw). Measurements. Elemental analyses were performed by means of a PerkinElmer CHNS/O PE2400 Series II. IR spectra (4000−400 cm−1) were recorded with a Bruker IFS55 FT−IR Spectrometer on KBr pellets. Emission spectra were collected with a Horiba-Jobin Yvon Fluoromax-4 Spectrofluorimeter using a DC xenon lamp. All spectra were spectrally corrected for detector response. Appropriate optical filters were used (475 long pass filter). Band pass was set as 5 nm slit width for each measurement. Electronic spectra were recorded with a Agilent Cary 5000 spectrophotometer. Solvent contribution was subtracted for all absorbance/ emission spectra. Electrochemical studies were performed with a Princeton Applied Research potentiostat/galvanostat model 263A equipped with a General Purpose Electrochemical System (GPES) in one-compartment three electrode cell, consisting of a platinum wire as the working electrode, a platinum wire as the counter-electrode, and Ag/AgCl (saturated KCl) as the reference electrode. The cyclic voltammograms were performed at room temperature (25 °C) in anhydrous and argon-degassed DMF containing 0.2 M tetrabutylammonium perchlorate (TBAClO4) at 100 mV/s; the ferrocenium/ferrocene couple presented E1/2 = 459 mV with an anodic to cathodic peak separation of 70 mV under the above conditions. The DC magnetic susceptibility of the compound was measured using a SQUID magnetometer (Quantum Design MPMS-XL 5T), with the sample in powder form embedded in a Teflon tape. The susceptibility was measured in CGS units in the temperature range of 2−350 K with an applied field of 10 kG. The susceptibility data were corrected for the diamagnetic contributions of the salt. X-ray Crystallography. A summary of data collection and structure refinement for (R)-Ph(Me)HC*-NMe3[PtL2]·DMF (1) is reported in Table 1. Single-crystal data were collected with a Bruker Smart APEXII area detector diffractometer, Mo Kα: λ = 0.710 73 Å. The unit cell parameters were obtained using 60 ω-frames of 0.5° width and scanned from three different zones of reciprocal lattice. The intensity data were integrated from several series of exposures frames (0.3° width) covering the sphere of reciprocal space.8 An absorption correction was applied using the program SADABS9 with minimum and maximum transmission factors of 0.622−1.000. The structure was solved by direct methods (SIR200410) and refined on F2 with fullmatrix least-squares (SHELXL-1411), using the WinGX software package.12 Non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were placed at their calculated positions. Graphical material was prepared with the Mercury 3.013 program. CCDC 1431286 contains the supplementary crystallographic data for this paper. Additional crystallographic information is available in the Supporting Information. Computational Details. The electronic properties of [PtL2]− were investigated by means of DFT methods using the spin-unrestricted formalism.14 All the calculations were performed with the Gaussian 03 program suite.15 The molecular structures of the complex were optimized starting from the X-ray experimental geometry of [PtL2]− without symmetry constraints. The Becke three-parameters exchange functional with Lee−Yang−Parr correlation functional (B3LYP)16,17 was employed together with the 6-31+G(d) basis set18,19 for the C, H, N, and S atoms. The Pt atom was treated with the SDD valence basis set20−22 and with the MWB60 (Pt) effective core potentials. Singlepoint calculations were performed using the B3LYP density fiunctional and the triple-ζ quality basis set def2-TZVP23 for the metal atoms and with pseudopotentials for Pt,24 whereas the C, H, N, S atoms were treated with the 6-311+G(d) basis set.25 TD-DFT calculations were performed for 30 excited states using the same level of theory of the single-point calculations. Natural population analysis (NPA) was

intense electronic transition falling at low energies and relatable to the highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) dipole-allowed transition; (2) monoanionic derivatives are paramagnetic and preserve the properties as NIR dye, so that they exhibit both magnetic and optical properties. In these complexes the b2g becomes a half-filled orbital, and the related HOMO−1 → singly occupied molecular orbital (SOMO) transition undergoes a bathochromic shift; (3) dianionic (and dicationic) derivatives are diamagnetic and commonly do not absorb in the NIR, since the full occupancy (and unoccupancy) of b1u and b2g results in a bleaching of the above-mentioned peak. For these reasons, electronic spectra and magnetic and electrochemical measurements provide suitable tools to assess the electronic structures in these complexes. Commonly, d8-metal homoleptic dithiolenes are not emissive, with the exception of Pt(II) complexes of quinoxaline-2,3-dithiolate and related ligands.6 The presence of the πacceptor quinoxaline ring adjacent to the dithiolene core (or connected through π-delocalized groups) favors the −2 as the most accessible oxidation state for homoleptic complexes. In the present work, a Pt(II) monoanion was obtained by employing the [4′,5′:5,6][1,4]dithiino[2,3-b]quinoxaline1′,3′dithiolato.7 The ligand exhibits a quinoxaline ring connected through thioether groups to the dithiolene core. The presence of thioether groups adjacent to the dithiolene core has a double function: on the one hand it interrupts the πcommunication of the quinoxaline ring with the dithiolene core; on the other hand, it also favors −1 as the most stable oxidation state for the complex. Accordingly, the platinum bisdithiolene monoanion was prepared, and by using the chiral cation C = [(R)-Ph(Me)HC*-NMe3]+, the C[PtL2] salt was isolated and structurally characterized. Tetra-alkyl ammonium salts of nickel and gold anionic complexes with the same ligand were previously synthesized, and the crystal structures of Bu4N[NiL2] and Bu4N[AuL2] were determined.7 These complexes were mainly investigated as NIR dyes and thirdorder nonlinear chromophores. In this paper, we report the preparation and the properties of the chiral C[PtL2] salt. The optical properties were investigated in solution, showing a peculiar proton dependence of the luminescence. Density functional theory (DFT) and time-dependent (TD) DFT studies were performed to provide a satisfactory structure/ property relationship.

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EXPERIMENTAL SECTION

Synthesis. Reagents and solvents were purchased from Aldrich and used without further purification. The ligand was prepared according to ref 7. [(R)-Ph(Me)HC*-NMe3][Pt(quinoxdt)2]·DMF (C[PtL2]·DMF) (1). Sodium (48 mg, 2.08 mmol) in 60 mL of MeOH was added dropwise to a stirred suspension of the ligand precursor (292 mg, 0.95 mmol) in the same solvent (50 mL). The mixture was stirred for 15 min at room temperature, and the solution turned from yellow to dark brown. To this solution, [Pt(DMSO)2Cl2] (200 mg, 0.47 mmol) dissolved in chloroform (120 mL) was added dropwise. The reaction mixture was then refluxed for 180 min under argon atmosphere, cooled under air, filtered, and the solvent evaporated to one-fourth of the initial volume. By addition of [(R)-Ph(Me)HC*-NMe3]I (275 mg, 0.95 mmol) the precipitation of a brown solid was obtained. After 60 min of stirring, this crude precipitate was collected by centrifugation and washed with diethyl ether (three times) and dried. Dark brown crystals (325 mg, 0.33 mmol; yield 79.0%) were obtained by recrystallization in dimethylformamide (DMF)/diethyl ether. Analytical results are in accordance with the formula [(R)-Ph(Me)HC*-NMe 3 ][PtB

DOI: 10.1021/acs.inorgchem.5b02491 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Summary of X-ray Crystallographic Data for (R)Ph(Me)HC*-NMe3[PtL2]·DMF empirical formula formula weight color, habit crystal size, mm crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z T, K ρ (calc), Mg/m3 μ, mm−1 θ range, deg No. of rflcn/obsv GOF R1 wR2

C34H33N6OPtS8 993.23 red, plate 0.32 × 0.27 × 0.13 triclinic P1 7.917(3) 9.545(4) 13.778(5) 75.536(6) 74.930(5) 76.395(5) 957.2(6) 1 293(2) 1.723 4.139 1.56 to 26.54 11 445/7838 1.010 0.0533 0.1351

Figure 1. Molecular structure of (R)-Ph(Me)HC*-NMe3[PtL2]·DMF with thermal ellipsoids drawn at the 30% probability level.

second ligand, Table 2. Consistently the C−S bond distances within the bent hexatomic ring are longer for the first ligand [1.78(2) and 1.82(2) Å] with respect those of the second ligand [1.75(2) and 1.77(2) Å]. Nevertheless, according to the three-sigma criterion, the bond distances are not significantly different. These types of systems were previously extensively investigated,29−31 and the overall electronic structure of the complex is generally represented by the following resonance forms: [PtII(L•−)(L−2)]− ↔ [PtIII(L−2)(L−2)]− ↔ [PtII(L−2)(L•−)]−, which take into account both the metal oxidation state and the overall charge on the dithiolate system. In the complex the bond distances exhibited by the ligands are in the range of those reported for analogous monoanionic nickel and gold complexes.7 Moreover, the Pt−S bond distances are in agreement with those reported for similar monoanionic square-planar platinum complexes.31b,c The molecular structure exhibits a bent shape due to the presence of the two thioether groups in the hexaatomic ring close to the dithiolate moiety. The peripheral aromatic systems of the two ligands are approximately parallel (the dihedral angle between the two mean planes formed by the aromatic groups is ∼2°), whereas the dihedral angle between the mean coordination plane and these peripheral aromatic systems is ∼40°. The crystal packing and relevant intermolecular interactions are reported in Figure 2 and Figure S1 in the Supporting Information. In particular, the assembly (R)-Ph(Me)HC*-NMe3[PtL2]·DMF forms supramolecular layers that run parallel to the b crystallographic axis. Each layer interacts with adjacent ones by means of CHmethyl··· N(12), CHmethyl···S(32), and CHphenyl···N(22) contacts. These contacts occur between the nitrogen and sulfur atoms of the ligand and the methyl and phenyl groups of the chiral cation. Within the layers the cation and the DMF molecules are sandwiched between two complex anions (Figure 2a) with which they exchange different interactions. In particular, the cation interacts with the peripheral part of one ligand by means of a partial π-stack (C(43)−C(51) = 3.33(3) Å) and by means of a C−H··· π contact (C(13)−C(21) = 3.77(3) Å), whereas the DMF molecule interacts with one of the ligands by means of a partial π-stack [C(1s)−C(32) = 3.31(4) Å]. The DMF

a R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2, w = 1/[σ2(Fo2) + (aP)2 + bP], where P = [max(Fo2,0) + 2Fc2]/3.

performed with the NBO 3.1 program26 incorporated in the Gaussian03 package. MO diagrams were generated with the Gabedit program.27 AOMix was used for determining atomic orbital compositions employing Mulliken population analysis.28

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RESULTS AND DISCUSSION Scheme 2 summarizes the synthetic procedure followed to obtain [PtL2]− complex in the radical-anion state, based on the [4′, 5′: 5, 6][1,4]dithiino[2,3-b]quinoxaline-1′,3′-dithiolate ligand. By using the chiral (R)-Ph(Me)HC*-NMe3+ as counterion, well-shaped crystals were grown from a DMF/ ether solution, and they were suitable for a X-ray crystallographic investigation. The molecular structure of (R)-Ph(Me)HC*-NMe3[PtL2]· DMF (1) is reported in Figure 1. The compound crystallizes in the chiral space group P1 according to the presence of the enantiopure cation (R)Ph(Me)HC*-NMe3+. In the lattice, a DMF molecule of crystallization is also present per each complex anion entity. In [PtL2]− the metal exhibits a square-planar geometry, and it is bound by four sulfur atoms from two bidentate S,S dithiolate ligands. The Pt−S bond distances are in the relatively narrow range of 2.267(6)−2.288(6) Å, whereas in the S−C−C−S fragments that comprise the formal dithiolate moieties, there is a greater variability in the bond distances. In particular, the fragment S(11)−C(11)−C(21)−S(21) of the first ligand presents C−S bond distances that are, on average, 0.06 Å shorter than the S(12)−C(12)−C(22)−S(22) system of the Scheme 2. Synthesis of (R)-Ph(Me)HC*-NMe3[PtL2]

C

DOI: 10.1021/acs.inorgchem.5b02491 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 2. Selected Bond Lengths (Å) and Angles (deg) for (R)-Ph(Me)HC*-NMe3[PtL2]·DMF, Together with the Optimized Goemetriesa of the Complex Anion [PtL2]− [PtL2]− distances X-ray Pt−S(11) Pt−S(21) Pt−S(12) Pt−S(22) C(11)−S(11) C(21)−S(21) C(12)−S(12) C(22)−S(22) C(11)−S(31) C(21)−S(41) C(12)−S(32) C(22)−S(42)

2.267(6) 2.268(6) 2.267(6) 2.288(6) 1.71(2) 1.72(2) 1.75(2) 1.77(2) 1.82(2) 1.78(2) 1.77(2) 1.75(2)

calculated

X-ray

2.336

C(31)−C(41) C(32)−C(42) C(11)−C(21) C(12)−C(22) C(41)−S(31) C(31)−S(41) C(42)−S(32) C(32)−S(42)

1.745

calculated

1.44(2) 1.45(2) 1.31(3) 1.33(3) 1.77(2) 1.79(3) 1.73(2) 1.74(2)

1.449 1.366 1.775

1.789

angles Pt−S(11)−C(11) Pt−S(21)−C(21) Pt−S(12)−C(12) Pt−S(22)−C(22) S(11)−Pt−S(21) S(12)−Pt−S(22) a

101.9(8) 102.1(6) 103.1(7) 102.0(6) 89.3(2) 89.7(2)

102.8

S(11)−C(11)−C(21) S(21)−C(21)−C(11) S(12)−C(12)−C(22) S(22)−C(22)−C(12)

124(2) 123(0) 123(2) 122.4(1)

122.8

88.4

For symmetry reason only the value of one geometric parameter is reported.

The composition of selected MOs is depicted in Figure S2. It can be appreciated that the SOMO is the out-of-plane antibonding combination between the dxz metal orbital and a π* ligand-centered orbital, having more than 55% 3pz sulfur character. The metal and sulfur atoms contribute to 11% and 60%, respectively, to the composition of the SOMO. Interestingly, the LUMO is represented by the π* orbital localized on the peripheral aromatic fragments of the ligands, at variance with similar metal dithiolate systems, in which the LUMO is mainly represented by an in-plane σ-antibonding interaction between the metal dxy or dx2−y2 and S 3pσ orbitals.30,32 This type of orbital is the LUMO+4 in the [PtL2]− complex, and a similar MO sequence was found for the dianion [Ni(pdt)2]−2 (pdt = bis(pyrazine-2,3-dithiolate)).33 The spin density is depicted in Figure S2c, and according to the NPA, it is mainly localized on each of the sulfur atoms (0.18) comprising the coordination environment, and it is also partially localized on the metal center (0.15), Table 3. Magnetic and Electronic Properties. The molar magnetic susceptibility (CGS units) versus temperature of a

Figure 2. Highlight on the interaction occurring within one supramolecular layer. Short intermolecular interactions are depicted as dashed lines. (a) DMF and cation sandwiched between two anions. (b) Depiction of the partial π stack occurring between the peripheral fragment of the ligands.

molecule and the cation interact by means of CHmethyl···O(1s) and CHmethyl···C(1s) contacts. Moreover, the complex anions give rise to a partial π-stack through the peripheral aromatic fragment of the ligands, and the minimum distance within this stack is exhibited by the C(51) and C(102) atoms [3.45(4) Å, Figure 2b]. Computational Studies. The complex [PtL2]− exhibits an S = 1/2 ground state (see magnetic and electronic properties section), and the calculations were performed with the spinunrestricted formalism. According to the geometric parameters reported in Table 2 there is a good agreement between the calculated and experimental geometries even though the metal−sulfur distances are slightly overestimated by the calculations (∼0.06 Ǻ ). The optimized structure is highly symmetric with the two ligand fragments having identical geometric parameters (bond distances and angles).

Table 3. Population Analysis and Spin Density of [PtL2]− in the Gas Phase (B3LYP/6-311+G(d)-def2-TZVP). Contribution of the Metal Center and of the Ligand Frameworka Are Reported

−

[PtL2]

a

D

Pt S N Cdithiol

Mulliken atomic charges 0.340 −0.853 0.144 0.612

NPA charges

NPA atomic spin density

Mulliken atomic spin density

0.336 −0.019 −0.452 −0.438

0.154 0.180 0.000 0.0275

0.100 0.150 0.001 0.0671

⟨S2⟩ 0.755

For symmetry reasons only the values of one sulfur is reported. DOI: 10.1021/acs.inorgchem.5b02491 Inorg. Chem. XXXX, XXX, XXX−XXX

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species. The irreversibility of this last process is likely due to the poor solubility of the neutral derivative in the used solvent. Preliminary CV experiments on 1 in CH3CN solution as a function of added p-toluenesulfonic acid suggest the exploration of the electrocatalytic potential of 1 for proton reduction from suitable sources.30 See Supporting Information. Optical Properties. Complex 1 is characterized in the NIR region by a broad absorption at 1085 nm in DMF with medium-high molar absorption coefficient [17.5 × 103 M−1 cm−1] and weaker absorptions in the visible region, as shown in Figure 4a. Very small spectral changes are observed on solvent variation (Figure S4). On increasing addition of I2 as oxidizing reagent to 1, the NIR peak at 1085 nm, typical of the monoanion, decreases, and a new one increaseas at 854 nm. A well-defined isosbestic point at 942 nm is observed (see Figure S5). This reflects an equilibrium between the monoanion and the neutral derivative. With the help of TD-DFT calculations, the observed absorption bands in 1 were assigned as shown in Figure 4b. The TD-DFT calculations were performed for 30 excitations, which are reported in Figure S6. Most of these excitations have negligible intensity, and in the visible region three bands can be identified at 449, 554, and 680 nm, whereas the most intense band falls in the NIR region at 1005 nm. As for other analogous monoanionic derivatives,29 the peak falling in the NIR is associated with a HOMO−1 → SOMO transition. The HOMO−1 consists of π-orbitals formed by in-phase combination of the S2C2S2 moieties (with antibonding features), whereas the SOMO consists of an antibonding combination of the metal [5dxz Pt orbital] and π-orbitals of the S2C2S2 system. It is noteworthy that no significant contribution of quinoxaline ring orbitals is provided to these orbitals. The two bands in the visible region at 680 and 554 nm were assigned as HOMO−1 → LUMO transitions, where the LUMO and LUMO+1 consist

polycrystalline sample of (R)-Ph(Me)HC*-NMe3[PtL2]·DMF is shown in Figure S3. The good fit with a simple Curie law and the value of the effective Bohr magneton number (p = 2.0 ± 0.1); the closest theorical value is p = 1.73) suggests a magnetic behavior typical of an uncoupled S = 1/2 ground-state species. This is in accordance with structural analysis, showing stacking motifs not favorable to induce magnetic interactions through πstacking between adjacent molecules (see Figure 2) as observed in similar complexes.34 Cyclic voltammograms (CV) of the complex were performed on anhydrous and argon-degassed DMF solutions (Figure 3).

Figure 3. CV of C[PtL2], c = 0.5 × 10−3 M in the presence of TBAClO4 0.2 M in DMF; working electrode: Pt; counter electrode: Pt; reference electrode: Ag+/Ag (saturated KCl), scan rate: 100 mV/s.

The CV exhibits two different waves: the reversible wave at E1/2 = −0.292 V (vs Ag/AgCl) (see the inset in Figure 3) corresponds to the reduction from [PtL2]− to the [PtL2]−2 species; an additional irreversible process can be associated with the formation of the neutral species after oxidation at +0.356 V. The reduction process at +0.208 V restores the monoanionic

Figure 4. (a) Absorption spectrum of 1 in DMF solution in the 400−1600 nm range. (b) TD-DFT, B3LYP/def2-TZVP_6-311+G(d) calculated spectra of the [PtL2]− anionic complex. (c) Calculated MOs involved in the absorption transitions. E

DOI: 10.1021/acs.inorgchem.5b02491 Inorg. Chem. XXXX, XXX, XXX−XXX

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The excitation band is relatable to the HOMO−1 → LUMO absorption transition (see Figure 4) involving orbitals of the quinoxaline moiety. This suggests that the emission is mainly ligand-centered in character without metal contribution, as also supported by the similarity of the emissive properties of the ligand precursor (see Figure S7). This may help to explain the unusual anti-Kasha behavior of this complex, showing emission at lower wavelengths than the lowest absorption band in the NIR region, which instead involves the orbitals of the dithiolene core (HOMO−1 → SOMO, Figure 4). It can be observed that the luminescence properties of 1 are similar (quantum yield, lifetimes) to those observed in analogous compounds, where no significant dependence on the metal (Ni, Pd, Pt) was found and the quinoxaline moiety can be considered as an ancillary ligand.36 In this regard, it can be also noticed that the estimated emission lifetime is actually comparable to those evaluated for internal conversion processes in similar dithione/dithiolate systems.37 As a final consideration, it should indeed be taken into account that when the energy gap between the upper excited states is sufficiently large, as in this case, exceptions to Kasha’s rule may be expected.38 The emission is proton-dependent, as shown in Figure 6a,b for HCl addition to 1 in acetone and DMF, respectively, in the 1:0 to 1:10 molar ratio, and the related quenching constants are of the same order of magnitude, though slightly higher for the acetone solution (see Figure S8). However, the vis/NIR absorption spectra of the corresponding solutions show small changes, if any, as reported in Figure S9. Similar behavior has been reported for the nickel complex Et4N[Ni(4-pedt)2] (4pedt = 1-(pyridine-4-yl)ethylene-1,2-dithiolate, where no shift of the NIR peak was observed when HClO4 is added in the 1:0 to 1:4 concentration ratio range.39 Being the NIR excitation mainly relevant to HOMO−1 → SOMO transition, it seems reasonable to propose that the energy gap is not affected noticeably by the protonation of the quinoxaline moiety, which does not provide significant contribution to these orbitals. The quenching process after excitation at 420 nm depends on the proton source, and this process is less effective with trifluoroacetic acid (TFA) than with HCl, with a quenching constant more than 1 order of magnitude lower (see Figures S8 and S10 in the Supporting Information). It was previously observed that the quenching process depends on the acid strength of the proton quenchers, and high concentrations of mild acid quenchers are required to suppress the emission.40 Moreover the lower effectiveness of TFA may also be due to possible self-association of the carboxylic acid.41 By using TFA in very strong excess with respect to 1, both a quenching of the luminescence, which undergoes a blue shift on increasing TFA excess, and a change in the absorbance spectra is observed, as shown in Figure 7. In particular, in the absorption spectrum, a decrease of the intensity of the peak at 1085 nm typical of the monoanion and the appearance of a peak at 854 nm is observed. This latter wavelength is predictable for the neutral species, and the presence of an isosbestic point connecting the two bands may suggest an equilibrium between these two species. However, the intensity of the peak at 854 nm is much lower than expected for the neutral derivative (see Figure S5). It is unclear whether this peak is relatable to the known neutral complex, which may be self-associated or to a new neutral form resulting from the protonation of the ligand. Apparently the solution seems clear, but with time the formation of a solid is observed, and this could tentatively justify the decrease in intensity of the

of π-orbitals of the quinoxaline and benzene-like moieties with no significant contribution of orbitals from the dithiolene core. Thus, HOMO−1 → LUMO transitions have charge-transfer character from the dithiolene core (S2C2S2 moieties) to the periphery of the ligand (quinoxaline moieties). The band at 449 nm is mainly associated with a HOMO−3 → LUMO+1 transition, and in this case the excitation requires a charge transfer from the metal and thioether sulfur atoms toward the peripheral quinoxaline ring. The experimental UV/vis/NIR spectrum shows a reasonable agreement with the simulated one. In fact, the NIR band falls at 1085 nm, whereas in the visible region, a broad band peaked at 445 with a weak shoulder around 550 nm and a band at 635 nm appear; see Figure 4a. While both HOMO−3 → LUMO+1 and HOMO−1 → LUMO transitions should contribute to the broad peak centered at 445 nm, we suggest that the main contribution is associated with the HOMO−1 → LUMO transition calculated at 554 nm, given its higher oscillator strength (Figure S6). The photoluminescence spectra of 1 were recorded in DMF and acetone solution as well as in the solid state. When excited at 420 nm at room temperature, these solutions showed emission at 572 nm (Figure 5 and Table 4). Instead, in the same experimental conditions, no emission was displayed by 1 in the solid state.

Figure 5. Emission (red), excitation (blue), and absorption (black) spectra of 1 in DMF solution. Excitation wavelength was 420 nm. The excitation spectrum was acquired by monitoring emission at 680 nm.

Table 4. Luminescence Parameters for 1 in Dimethylformamide or Acetone Solutions C[PtL2]

λex

λem

solvent

Φ

420 420

566 572

acetone DMF

0.0011 0.0019

The emission quantum yields, 0.11% in acetone and 0.19% in DMF, were evaluated using the relative method, as described in Supporting Information and summarized in Table S1, by employing [Ru(bpy)3]Cl2 as suitable reference,35 given the similarity of its absorption/emission spectral ranges with respect to 1. A rough estimation of emission lifetime (through quantum yield and spectral data; see Supporting Information) yields a value on the order of tens of picoseconds. F

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Figure 6. Variation of the emission spectrum of 1 in acetone solution (a) and DMF solution (b) on increasing HCl concentration. Excitation wavelength was 420 nm.

Figure 7. (a) Absorption spectra for 1 in DMF solution (1 × 10−4 M) on variation of TFA concentration. (b) Variation of emission intensity of 1 in DMF solution on increasing TFA concentration. Excitation wavelength was 420 nm.

new band at 854 nm and related to the neutral self-associated [PtL2] that precipitates out of the solution. As far as the visible bands are concerned, the modification of the absorption spectrum upon acid addition (red shift of the visible bands) is fully justified by considering the nature of the electronic transitions. In fact, these excitations require a charge transfer from the central core of the complex toward the peripheral quinoxaline ring (LUMO and LUMO+1), whose nitrogen atoms are also the preferable protonation sites of the complex. In fact, by inspecting the electrostatic potential (EP) mapped on the isodensity surface of the complex, it is evident that the nitrogen atoms accumulate greater negative charge than the sulfur atoms of dithiolene core, Figure 8. The charge distribution in 1 obtained from the NPA (see Table 3 and Figure S11) is also in agreement with EP map. Thus, the first protonation sites of 1 upon acid addition involve N atoms in agreement with emission quenching experiments upon HCl addition. No shift in the emission signal is observed, and this should be consistent with a dynamic quenching process. Instead using TFA in very strong excess with respect to 1, both

Figure 8. Electrostatic potential for [PtL2]−·mapped on the isodensity surface. Color codes thresholds: red −0.15, yellow −0.1, green −0.05, light blue −0.0, blue 0.2.

a quenching of the luminescence accompanied by a blue shift and a change in the absorbance spectra is observed. In this case both N protonation and possibly proton transfer to the dithiolenic core may occur. G

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Inorganic Chemistry In conclusion the employed ligand is capable to favor the −1 as the most accessible state for the complex. The presence of the thioether groups adjacent to dithiolene core interrupts the π-communication of quinoxaline ring with the dithiolene core. At the same time, the heteroaromatic quinoxaline moiety provides both luminescent properties as well as the supramolecular architectures through π−π interactions and Hbonding. Moreover, the π−π contacts in 1 do not affect its magnetic properties, even though they seem to be responsible for the loss of luminescence of the solid sample. The luminescent properties of this complex are highly unusual, since the emission (560 nm) falls well-above the energy of the lowest energy absorption (1085 nm). A thorough photophysical study, including transient absorption experiments, will be crucial to correctly clarify the photocycle in this complex and to definitely assess the nature of the emissive species that can possibly be photogenerated in solution. The features of 1, which is also redox-active, seem promising to be further explored in regard to photo- and/or electrocatalytic activity for proton reduction in acidic aqueous solution.42,43

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02491. Additional crystallographic data can also be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. The molecular structure and MOs calculations, magnetic properties, additional absorption and photoluminescence spectra, photophysical parameters evaluation, electronic properties, electrocatalytic activity. (PDF) X-ray crystallographic data. (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (L.M.) *E-mail: [email protected]. (P.D.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Authors warmly thank Prof. A. Cannizzo for helpful discussion regarding anti-Kasha behavior. This research was supported by Fondazione Banco di Sardegna and Università di Cagliari. Thanks are also due to COST Action CM1202, PERSPECT-H2O Supramolecular photocatalytic water splitting.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02491 Inorg. Chem. XXXX, XXX, XXX−XXX