Tuning the Phosphorescence and Solid State Luminescence of

Article pubs.acs.org/IC

Tuning the Phosphorescence and Solid State Luminescence of Triarylborane-Functionalized Acetylacetonato Platinum Complexes George Rajendra Kumar and Pakkirisamy Thilagar* Inorganic and Physical Chemistry Department, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: A new series of luminescent cyclometalated platinum complexes with triarylborane-functionalized acetylacetonate ligands is reported. The complexes exhibit solid state luminescence and phosphorescence under ambient conditions. The luminescence color can be tuned from green to red by varying the cyclometalating ligand [2-phenylpyridine (for 1 and 2), 2-thiophenylpyridine (for 3 and 4), 2-thianapthenylpyridine (for 5 and 6)]. The luminescence originates from mixed 3 MLCT/3IL [MLCT, metal to ligand charge transfer; IL, intraligand] states of square planar platinum and borane moieties. The π spacer (phenyl or duryl) which connects the boryl and platinum entities has a significant role in determining the photoluminescence efficiency. The bulky duryl spacer in 2, 4, and 6 significantly reduces π−π stacking of the square planar platinum moiety in the solid state and provides a rigid backbone, thereby increasing their quantum yield significantly. The role of Lewis-acidic borane on the photoluminescence features is evaluated by fluoride binding experiments.

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INTRODUCTION Organometallic luminescent materials have elicited great interest in the last two decades owing to their potential applications in modern technologies such as solid state lighting,1 biological applications,2−4 and detection of various analytes.5 They have also been used as light-harvesting antenna for selective sensitization of lanthanide luminescence.6 Among them, square planar platinum [Pt(II)]-based phosphorescent materials are well known for their tunable optical characteristics.7 As a heavy metal, Pt(II) induces spin−orbit coupling when introduced into an organic backbone and effectively populates the triplet emissive state with a long lifetime.8 It is demonstrated that cyclometalation is an efficient method for tuning phosphorescence in Pt(II) complexes.9 Through a proper manipulation of ligand structure, these complexes can be exploited for various applications such as OLED and bioimaging.10 Integrating versatile functional groups with Pt(II) complexes is of particular interest for tuning their luminescence characteristics.7−10 Among the various methods, incorporation of a triarylborane (TAB)11 moiety in a phosphorescent system is a promising tool for developing optoelectronic materials as the material can serve as both emissive as well as electron transporting layer in OLEDs.12 In this regard, Wang and coworkers developed a series of TAB-conjugated Pt(II) complexes.12c−e,g−i In most of the cases, cyclometalating ligands are functionalized with a TAB moiety, and reports on TABfunctionalized ancillary ligands remains rare.13 In this paper, we report Pt(II) complexes with triarylborane (TAB)-conjugated © XXXX American Chemical Society

acetylacetone (acacH) as ancillary ligands based on our previous experience with boryl-functionalized luminophores and BF2−diketones.14 We envisioned that if the TABfunctionalized acacH is used as an ancillary ligand, it would be readily possible to alter cyclometalating ligand structure, which is the key component of luminescence in these Pt(II) complexes. Furthermore, the bulky Lewis-acidic TAB moiety can reduce intermolecular interactions and enhance MLCTbased emission. Three cyclometalating ligands (2-phenylpyridine, 2-thiophenylpyridine, and 2-thianapthenylpyridine) are chosen in order to tune the phosphorescence color of Pt(II) complexes.

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic route for TAB-conjugated Pt(II) complexes (1−6) is shown in Scheme 1. Heating a mixture of (Mes)2−π-spacer−acetylacetone (A, spacer = phenyl; B, spacer = duryl) with μ-chlorobridgedcyclometalated platinum dimers in dry acetonitrile along with sodium carbonate under nitrogen atmosphere resulted in the targeted platinum complexes (1−6) in good yield (50−80%). The crude products were purified by column chromatography over silica gel. The complexes are characterized by ESI-mass, elemental analysis, and NMR spectroscopic methods. Formation of cyclometalated complexes is supported by the disappearance of the signal arising from the enol proton of the Received: July 30, 2016

A

DOI: 10.1021/acs.inorgchem.6b01827 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthetic Scheme for the Preparation of 1−6

Figure 1. Molecular structures of 2 (left), 3 (middle), and 4 (right). Thermal ellipsoids are shown at the 30% level.

Figure 2. Packing diagrams of 2 (left) and 3 (right) in the solid state.

Figure 1. The Pt(II) centers adopt a square planar geometry, and the boron centers display trigonal planar geometry. The π spacer (C6H4 and C6Me4) plays a crucial role in arrangement of boryl and square planar Pt(II) entities with respect to each other. In the case of 3, the dihedral angle between the planes of square planar platinum and trigonal planar boron is 54.55°, but

acetylacetone moiety and the appearance of a new set of peaks corresponding to cyclometalated rings in the 1H NMR spectrum. Molecular Structures. The molecular structures of compounds 2, 3, and 4 were confirmed by the single-crystal X-ray diffraction technique.15 The structures are shown in B


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Inorganic Chemistry

Figure 3. UV−vis absorption (left, under N2) and photoluminescence spectra of complexes 1−6 in CH2Cl2 (middle, under N2, peak at around 640 nm corresponds to second-order diffraction of excitation light) and in the solid state (right, under open atmosphere) at 298 K. (Insets) Photographic images of complexes 1−6 in solution and the solid state under UV light.

Table 1. Photophysical Properties of Complexes 1−6 solution state (concentration = 1 × 10−5 M) complexes

λabs/nm (ε × 10−4/M−1 cm−1)

λem/nm

τa/μs

ΦPL (N2/O2)

solid statea λem/nm

τa/μs

ΦPL

0.20/0.076

528

0.23

0.40/0.083 0.85/0.032 0.85/0.029

530 574 564

τ1 = 1.55 (A = 92.8%) τ2 = 10.33 (A = 7.2%) 10.57 2.43 τ1 = 360 (A = 18.4%) τ2 = 4.07 (A = 81.6%) τ1 = 2.65 (A = 85.1%) τ2 = 8.50 (A = 14.9%) τ1 = 7.74 (A = 24.9%) τ2 = 3.14 (A = 75.1%)

#

1

265 (3.16), 280 (1.97), 322 (3.08), 368 (1.07), 400 (0.47)

482

0.23

2 3 4

280 (3.05), 317 (2.96), 329 (3.17), 372 (1.01), 405 (0.37) 266 (2.44), 297 (2.37), 320 (2.82), 362 (0.90), 406 (0.42), 425 (0.34) 289 (1.98), 319 (2.65), 334 (2.97), 367 (0.87), 405 (0.44), 426 (0.34)

484 556 556

0.27 0.58 0.54

5

265 (2.86), 317 (3.66), 370 (0.73), 426 (0.53), 447 (0.51)

606

0.52

0.13/0.019

613

6

287 (1.85), 322 (2.78), 333 (2.68), 375 (0.62), 425 (0.50), 448 (0.48)

606

0.53

0.17/0.017

614

#

0.58 0.35 0.40 0.14 0.26

a

Measured under open atmospheric condition. Quantum yields are measured using calibrated integrating sphere. A = amplitude. All studies are carried out under room-temperature condition. # PL quantum yield of 2 is nearly double the value observed for 1 under N2 atmosphere; with the available data, it is hard to explain this anomalous behavior of the former.

absorption spectrum displays a characteristic boryl-based absorption band (spanning between 300 and 350 nm) which is ascribed to π to pπ* transition in all cases. Small shoulder peaks appearing at higher energy wavelength regions are ascribed to mixed π to π* transitions of mesityl and other aromatic units. Complexes 1 and 2 (C^N; 2-phenylpyridine) exhibit a weak absorbance at ∼370 nm, and it tails up to 420 nm. This low-energy absorption band can be attributed to combinations of MLCT, LLCT, and IL (intra ligand) transitions. This low-energy absorption band shows considerable change when 2-phenylpyridine is replaced with other cyclometalating ligands such as 2-thiophenylpyridine and 2thianapthenylpyridine. In the case of 3 and 4 (C^N; 2thiophenylpyridine), additional low-energy bands are noted at ∼405 and ∼425 nm. On the other hand, in the case of 5 and 6 (C^N; 2-thianapthenylpyridine), these low-energy bands undergo a red shift (∼425 and ∼440 nm). These observations clearly indicate that unlike 1 and 2, complexes 3, 4, 5, and 6 show additional low-energy transitions. Probably, the presence of a heteroatom, sulfur, may induce ligand-centered (LC) π−π* transitions at the low-energy region. A prominent red shift of these absorption bands in 5 and 6 may be due to the cooperative effect from a more π-conjugated planar thianapthenyl ring. For compound 5 the observed molar extinction coefficient of the peak at 320 nm is significantly higher than the value observed for 6. TD-DFT results (Supporting Information, (vertical excitation energies of singlet states of 5 and 6)) show

the same in compounds 2 and 4 are 31.7° and 32.9°, respectively. However, the dihedral angles between the planes of the π spacer and the square planar Pt(II) moiety in 2, 3, and 4 are nearly the same (87.5°, 86.5°, and 87.7°, respectively). On the contrary, the dihedral angles between the π spacer and the trigonal planar boron moiety in 2, 3, and 4 (56.1°, 31.4°, and 55.1°, respectively) vary significantly. Solid state packing patterns of 2 and 3 differ from each other (Figure 2). In 3, the molecules are arranged in a dimeric fashion with intermolecular interactions between the square planar Pt(OÔ)(C^N) moieties with a distance of 3.703 Å between neighboring thiophenylpyridine units. Though the complex with phenyl spacer (3) forms dimeric structure in the solid state, no metal−metal interaction is observed. On the other hand, 2 shows a different packing pattern. The bulky duryl spacer in 2 restricts the formation of dimeric (π−π interaction) structures. Complex 4 also follows a similar pattern as that of 2. It is well known that solid state luminescence can be controlled or tuned by structural arrangements.16 From molecular structural analysis, it is clear that the complex with a duryl spacer (2) is highly rigid and shows dissimilar intermolecular interactions compared with the one (3) having a phenyl spacer. Hence, one can expect the complex with a duryl spacer to show a higher luminescence quantum yield. Optical Properties. UV−vis absorption and emission features of complexes 1−6 in CH2Cl2 at room temperature are shown in Figure 3, and the data are listed in Table 1. The C


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Inorganic Chemistry

by comparing the photophysics of complexes 1−6 under investigation with those of 7−9. Quantum yields of complexes 1−6 are significantly higher than those of 7−9. The excited state lifetime for complexes 1−6 is slightly shorter than that for 7−9. Introduction of the TAB group at the third carbon of the acacH unit of platinum complexes increases their luminescence efficiency without altering the emission wavelengths. This observation confirms that the TAB group has a significant role in controlling the luminescence efficiency of 1−6. Complexes 1−6 exhibit bright luminescence in the solid state (Figure 3) with an appreciable quantum yield and with emission color tunability from green to red. In particular, complexes with a duryl spacer (2, 4, and 6) exhibit slightly higher emission compared to the ones having a phenyl spacer (1, 3, and 5). Indeed, the bulky duryl spacer enhances the emission efficiency, which can be a result of its effect on the molecular structure. Evidently the molecular arrangement in the solid state plays a crucial role in tuning the emission efficiency. The quantum yield values are significantly higher than the solution state which may be due to the absence of strong π−π interactions and metal−metal interaction in the solid state. Moreover, the solid state luminescence emission maxima show a slight red shift compared to solution state (Supporting Information, Figure S5). Such a red shift can be attributed to the intermolecular interactions in the solid state, which may alter the electronic energy levels and lower the cumulative band gap.19 A prominent red shift in the case of 1 and 2 suggests that 2-phenylpyridine favors such kind of intermolecular interactions to a larger extent. To confirm the origin of the solid state emission, time-resolved photoluminescence studies were carried out for solid samples in the presence of air. Except compounds 2 and 3 all other compounds show a biexponential decay profile. Overall a longer photoluminescence lifetime on the order of microseconds was observed for complexes 1−6 compared to their respective solutions (Table 1). These results clearly indicate that in the solid state 1−6 prefer to relax via a radiative process. Thus, the observed PL quantum yield of compounds 1−6 is significantly larger than the value observed for the respective solutions in the presence of air. These results ascertain that complexes 1−6 have the potential to act as good phosphors in the solid state. DFT and TD-DFT Computational Studies. Gaussian 09 is used for geometry optimization with B3LYP as the hybrid functional (considering the LANL2DZ basis set for platinum and 6-31G* for all other atoms).20 The computed molecular orbitals (MOs) are depicted in Figure 4. The optimized structures corroborate well with structures determined by X-ray diffraction. HOMO−1 of all complexes distribute in a similar manner with a major contribution from the acetylacetonato ring. In all cases, HOMOs are localized over the π orbitals of the C^N ligand (phenyl and thienyl) and platinum’s d orbitals. In the case of 5 and 6 the contribution of the acetylacetonato ring to the HOMO is significantly less compared to the other compounds. The localization of the LUMO and LUMO+1 varies significantly. In the case of complexes 1 and 3 with phenyl spacer, the LUMO is localized on the TAB unit with a significant contribution from the empty p orbital of the boron center, and the LUMO+1 is localized on the π* orbital of the C^N unit with a major contribution from the pyridine unit and a minor contribution from platinum’s d orbital. In the case of 5, LUMO and LUMO+1 are localized equally on both TAB unit as well C^N units. On the other hand, complexes 2, 4, and 6

that in the case of 5 the band at 320 nm has a considerable contribution from boron compared to 6. Thus, the involvement of boron enhances the delocalization of orbitals in 5, which may be the possible reason for the observed larger absorbance. All six complexes exhibit structured emission bands when excited at ∼320 nm. The emission bands undergo a red shift (from 450 to 600 nm) on going from 1, 2 to 5, 6 (for 1 and 2, 482 nm; for 3 and 4, 556 nm; for 5 and 6, 606 nm). This observation indicates that the origin of the emissive states in these complexes must be a combination of the Pt(II) metal center and cyclometalating ligands. The structured emission observed clearly suggests the domination of intraligand (IL) π−π* transitions over the (MLCT) transition in the emissive state.17 The excitation spectrum (Supporting Information, Figures S1−S3) of each of the complexes resembles its absorption spectrum, which further confirms the involvement of IL-, LLCT-, and MLCT-based excited states in the emission process. Complexes with a duryl spacer show slightly more intense emission bands compared to those having a phenyl spacer (except 3 and 4, having almost equal quantum yield). Moreover, the quantum yields of complexes 1−6 are significantly higher than the platinum complexes with triarylboryl-functionalized dibenzoylmethane ligands reported elsewhere.12 The luminescence intensity of the complexes in the presence of oxygen is reduced considerably (Supporting Information, Figure S4), and a decrease in quantum yield is observed. The heavy metal [Pt(II)] effectively induces spin− orbit coupling and facilitates intersystem crossing, thereby populating the triplet excited state. The photoluminescence lifetime for complexes 1−6 lies in the range of 0.2−0.6 μs and confirms the involvement of the triplet excited state in the emissive state. These results support the earlier inference that the emission originates from a triplet excited state (T1) which is sensitive to atmospheric oxygen. Moreover, nearly the same PL quantum yield was observed for 1 and 2 in the presence of air. The similar excited state lifetime noted for 1 and 2 is also in line with the above observations. The same trend is noted in other complexes (3−6) as well. In 2002, Thompson and co-workers reported a series of cyclometalated platinum complexes with simple acacH as the ancillary ligand and 2-phenylpyridine (7), 2-thiophenylpyridine (8), and 2-thianapthenylpyridine (9) as cyclometalating ligands (Chart 1, Table 2).18 The following conclusions can be drawn Chart 1. Cyclometalated Platinum Complexes without a TAB Group

Table 2. Photophysical Data for 7−9 at 298 K in 2Methyltetrahydrofuran complexes

λmax/nm (PL)

Φ

τ/μs

7 8 9

486 575 612

0.15 0.11 0.08

2.6 4.5 3.4

D


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Inorganic Chemistry

Figure 4. Molecular orbital diagrams of 1−6 (iso value = 0.04).

Calculated vertical excitation energies for the first 12 excited states (both singlet and triplet) are provided in the Supporting Information (page S10−S26). In 1 and 3, both S1 and T1 states are dominated by a HOMO to LUMO+1 transition. This observation clearly suggests that S1 and T1 states of 1 and 3 involve mixed IL π to π* and MLCT transitions. The S2 states of 1 and 3 arise as a result of HOMO to LUMO and HOMO− 1 to LUMO transition. On the other hand, T2 states are dominated by HOMO−1/HOMO to LUMO+1 transition. The T4 states of 1 and 3 have a significant contribution from HOMO−2 to LUMO. In the case of complex 5, S1, S2, T1, and T2 states are mostly dominated by transitions from lower energy orbitals (HOMO, HOMO−1) to LUMO and LUMO +1. Hence, in 5, the lower energy transitions can be assigned as a mixture of MLCT, IL π to π*, and LLCT transitions. Similarly, in complexes 2, 4, and 6, S1 and T1 states are dominated by the HOMO to LUMO transition. The HOMO to LUMO+1 transition contributes to the S3 state of 2 and 4 and S2 state of 6. The T5 state of 2 and 4 and T6 state of 6 predominantly arise as a result of transitions from lower energy HOMOs to LUMO+1. These results confirm that the Lewisacidic boron center together with C^N ligands play an important role in MLCT- and LLCT-based transitions. These results clearly indicate that the higher luminescence quantum yields observed for 1−6 are due to the direct influence of the empty p orbital of boron on the emissive excited states. These results are in line with the previously reported literature.12c Effect of Fluoride Binding on Photoluminescence. It is well established that the boron center in TAB can detect fluoride ions.21 Anion binding to the boron center leads to modification of the electronic conjugation and causes distinct changes in the photophysical character of the respective compounds. Hence, to evaluate the role of the electrondeficient Lewis-acidic boron center on the luminescence behaviors, complex 1 is titrated against tetrabutylammoniumfluoride (TBAF) in CH2Cl2 medium under nitrogen atmosphere (Figure 6). Upon addition of TBAF to 1 the absorption band at ∼320 nm gradually decreased and became saturated after the addition of 1 equiv of fluoride ions. Binding of fluoride ion to the boron center stops the π−pπ* transition of the boryl moiety and results in quenching of the boryl-based absorption band. The lower energy absorption band (∼380 nm) undergoes a slight

with a duryl spacer follow a different trend. The LUMOs are localized on the π* orbitals of the C^N ring with a major contribution from the pyridine unit, and LUMO+1 are localized on TAB with a significant contribution from the boron empty p orbital. Apparently, the bulky duryl spacer significantly affects the distribution of LUMO and LUMO+1. The sterically encumbered duryl spacer restricts the mixing of orbitals of boryl and C^N units. The energies of four primary molecular orbitals (MOs) of complexes 1−6 are depicted in Figure 5. Introduction of a heteroatom (sulfur) in the

Figure 5. Plots of complexes vs their molecular orbital energies.

cyclometalating ligand alters the energies of the frontier molecular orbitals. In the case of complexes 3−6, the HOMO−LUMO gap is reduced due to the presence of electron-rich thiophenyl and thianapthenyl moieties. The presence of a heteroatom and fused phenyl moiety increases the π conjugation within the cyclometalating ligands. These observations clearly support the inference that the low-energy absorption bands (>340 nm) in 1−6 have a significant contribution from the Pt(C^N) moiety.18 Though the localization of the LUMO and LUMO+1 orbitals of complexes with a phenyl and duryl spacer vary, no deviation in the lower energy absorption band (assigned as MLCT, LLCT, and IL) and emission band are noted. Hence, in order to obtain more insight into the photophysical properties of complexes 1−6, TD-DFT calculations were performed. E


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Inorganic Chemistry

Figure 6. UV−vis absorption (left) and photoluminescence (right) changes of 1 in CH2Cl2 at 298 K under N2 atmosphere (1 × 10−5 M, λex = 315 nm) in the presence of TBAF (2 μL = 0.1 equiv).

Figure 7. HOMO (left) and LUMO (right) orbitals of 1·F¯.

Figure 8. Molecular orbital energies of selected parts of 1 and 1·F¯.

calculation supports that the lowest energy transition in 1·F− occurs from the HOMO to the LUMO (in both singlet and triplet states). Hence, the lowest excited state in 1·F− must be a dark state, and the transition from T1 to S0 is nonemissive in nature. To further rationalize the above concept, molecular orbital energies for individual molecular entities of 1 and 1·F− [BMes2(C6H5), BFMes2(C6H5)−, and phenylacetylacetonatoplatinumphenylpyridine (M1)] are calculated. In the case of BMes2(C6H5), the HOMO is localized on the π orbitals of the mesityl rings, while in BFMes2(C6H5)− the HOMO is delocalized over mesityl and phenyl rings with a significant contribution from B−C σ bonds. On the other hand, the HOMO of M1 has a contribution from π orbitals of the C^N unit, acac unit, and d orbitals of platinum(II) (Supporting Information, Figures S7−S9). The HOMO of dimesitylphenylborane lies below the HOMO of M1. However, after fluoride binding the energy of the HOMO is raised above the HOMO level of M1 (Figure 8). This observation directly

red shift (∼5 nm). At the same time, the photoluminescence intensity of 1 is quenched to a greater extent by fluoride addition. Fluoride titration experiments show that fluoridebound tetracoordinated boron reduces luminescence intensities in all cases. As discussed vide supra, the empty p orbital of the boron center is significantly involved in the emissive excited states of 1−6. Thus, the fluoride binding at the boron center modifies the coordination and electronic structure of the boryl unit and thereby quenches the luminescence. This observation is in line with previously reported molecules.12c,13 To understand the mechanism of emission quenching upon fluoride binding, the fluoride adduct 1·F− was optimized by the DFT method (Figure 7). Fluoride binding leads to abrupt changes in localization of MOs. The HOMO is localized on mesityl rings, while the LUMO is localized on the π* orbital of pyridine and d orbitals of Pt(II). The specific localization of MOs of 1·F−, which differs completely from 1, suggests the possibility of the existence of photoinduced electron transfer (PET) from Ar3B·F− to the cyclometalated ring. TD-DFT F


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Inorganic Chemistry

J = 4 Hz, 2H), 7.52 (d, J = 8 Hz, 2H), 7.46 (d, J = 8 Hz, 1H), 7.21 (m, 3H) 7.11 (m, 2H) 6.83 (s, 4H), 2.31 (s, 6H), 2.03 (s, 12H), 1.77 (s, 6H). 13C NMR (100 MHz, CDCl3, δ ppm) 184.96, 169.03, 147.69, 145.49, 145.19, 141.23, 140.81, 139.64, 139.13, 138.52, 137.22, 136.34, 133.69, 131.98, 131.17, 129.74, 129.20, 128.68, 124.01, 123.47, 118.80, 30.16, 23.84, 21.67. ESI mass calcd 773.2873; obsd [M + H+] 773.2909. Anal.Calcd for C40H40BNO2Pt: C, 62.18; H, 5.22; N, 1.81. Found: C, 62.48; H, 5.13; N, 1.80. Complex 2. B (0.130 g, 0.2709 mmol), Na2CO3 (0.07 g, 0.645 mmol), platinum dimer (0.1 g, 0.129 mmol), acetonitrile (25 mL). Yield: 0.067 g (62%) of straw yellow color solid. 1H NMR (400 MHz, CDCl3, δ ppm) 9.05 (d, J = 4 Hz, 1H), 7.80 (t, J = 4 Hz, 1H), 7.69 (d, J = 4 Hz, 1H), 7.64 (d, J = 8 Hz, 1H), 7.47 (d, J = 8 Hz, 1H), 7.23 (m, 1H), 7.12 (m, 2H) 6.76 (s, 4H), 2.27 (s, 6H), 1.99−2.07 (m, 24H), 1.65 (s, 6H). 13C NMR (100 MHz, CDCl3, δ ppm) 184.90, 168.99, 147.69, 145.05, 141.35, 141.30, 141.18, 141.10, 140.77, 139.65, 138.56, 136.32, 133.67, 131.07, 129.82, 129.38, 129.32, 129.20, 124.02, 123.50, 121.63, 118.83, 115.59, 27.53, 23.72, 23.25, 21.70, 20.98, 17.25. ESI mass calcd 829.3499; obsd [M + H+] 829.3455. Anal. Calcd for C44H48BNO2Pt: C, 63.77; H, 5.84; N, 1.69. Found: C, 63.70; H, 5.65; N, 1.81. Complex 3. A (0.114 g, 0.269 mmol), Na2CO3 (0.084 g, 0.7940 mmol), platinum dimer (0.1 g, 0.128 mmol), acetonitrile (25 mL). Yield: 0.07 g (71%) of yellow color solid. 1H NMR (400 MHz, CDCl3, δ ppm) 8.81 (d, J = 8 Hz, 1H), 7.68 (t, J = 8 Hz, 1H), 7.51 (m, 3H), 7.31 (d, J = 8 Hz, 1H), 7.20 (m, 3H), 6.92 (t, J = 4 Hz, 1H), 6.83 (s, 4H), 2.31 (s, 6H), 2.02 (s, 12H), 1.77 (s, 3H), 1.73 (s, 3H). 13C NMR (100 MHz, CDCl3, δ ppm) 184.29, 182.68, 164.79, 147.58, 145.33, 144.34, 142.26, 141.23, 139.46, 139.15, 137.23, 131.93, 130.68, 128.68, 128.23, 118.88, 117.73, 117.63, 30.15, 29.49, 23.84, 21.68. ESI mass calcd 779.2437; obsd [M + H+] 779.1750. Anal. Calcd for C38H38BNO2PtS: C, 58.61; H, 4.92; N, 1.80. Found: C, 58.52; H, 4.86; N, 1.71. Complex 4. B (0.129 g, 0.268 mmol), Na2CO3 (0.094 g, 0.889 mmol), platinum dimer (0.1 g, 0.127 mmol), acetonitrile (25 mL). Yield: 0.05 g (51%) of yellow color solid. 1H NMR (400 MHz, CDCl3, δ ppm) 8.86 (d, J = 4 Hz, 1H), 7.66−7.71 (m, 1H), 7.51 (d, J = 4 Hz, 1H), 7.32 (d, J = 4 Hz, 1H), 7.25 (s, 1H), 6.93 (t, J = 8 Hz, 1H), 6.75 (s, 4H), 2.27 (s, 6H), 2.03 (s, 12H), 1.99 (s, 12H), 1.65 (s, 3H), 1.61 (s, 3H). 13C NMR (100 MHz, CDCl3, δ ppm) 184.07, 182.28, 164.45, 156.36, 147.27, 144.74, 144.13, 141.03, 140.77, 140.29, 139.39, 138.82, 136.03, 133.29, 130.27, 129.01, 128.90, 128.03, 118.58, 117.32, 27.96, 26.80, 23.41, 22.93, 21.38, 20.66, 16.89. ESI mass calcd 835.3063; obsd [M + H+] 835.2950. Anal. Calcd for C42H46BNO2PtS: C, 60.43; H, 5.55; N, 1.68. Found: C, 60.32; H, 5.38; N, 1.75. Complex 5. A (0.101 g, 0.2382 mmol), Na2CO3 (0.084 g, 0.7938 mmol), platinum dimer (0.1 g, 0.1134 mmol), acetonitrile (25 mL). Yield: 0.075 g (80%) of yellow solid. 1H NMR (400 MHz, CDCl3, δ ppm) 8.90 (d, J = 2 Hz, 1H), 8.78 (m, 1H), 7.82 (m, 1H), 7.70 (t, J = 4 Hz, 1H), 7.56 (d, J = 8 Hz, 2H), 7.33 (m, 2H), 7.28 (m, 1H), 7.25 (m, 2H), 6.92 (t, J = 8 Hz, 1H), 6.81 (s, 4H), 2.33 (s, 6H), 2.06 (s, 12H), 1.84 (s, 3H), 1.78 (s, 3H). 13C NMR (100 MHz, CDCl3, δ ppm) 184.50, 182.14, 165.18, 153.76, 147.30, 145.85, 145.42, 145.02, 142.77, 142.26, 141.25, 140.63, 139.98, 139.20, 137.29, 131.96, 128.73, 127.13, 125.85, 124.60, 122.84, 119.26, 118.65, 117.71, 30.18, 29.40, 23.89, 21.72. ESI mass calcd 829.2593; obsd [M + H+] 829.2650. Anal. Calcd for C42H40BNO2PtS: C, 60.87; H, 4.86; N, 1.69. Found: C, 60.71; H, 5.01; N, 1.89. Complex 6. B (0.114 g, 0.238 mmol), Na2CO3 (0.084 g, 0.7938 mmol), platinum dimer (0.1 g, 0.1134 mmol), acetonitrile (25 mL). Yield: 0.073 g (73%) of yellow color solid. 1H NMR (400 MHz, CDCl3, δ ppm) 8.95 (d, J = 4 Hz, 1H), 8.85 (m, 1H), 7.75−7.82 (m, 2H), 7.73 (d, J = 8 Hz, 1H), 7.34 (d, J = 8 Hz, 1H), 7.20−7.23 (m, 1H), 6.98 (t, J = 4 Hz, 1H), 6.76 (s, 4H), 2.28 (s, 6H), 1.99−2.05 (m, 24H), 1.73 (s, 3H), 1.69 (s, 3H). 13C NMR (100 MHz, CDCl3, δ ppm) 184.36, 182.02, 165.29, 150.19, 147.36, 145.88, 145.29, 145.01, 142.85, 140.32, 140.11, 137.06, 136.43, 133.66, 129.33, 129.23, 127.07, 125.88, 125.48, 124.95, 121.55, 120.04, 119.29, 118.69, 30.15, 28.15, 23.71, 23.23, 21.68, 20.97, 17.25. ESI mass calcd 885.3219; obsd [M +

supports the probability of the PET process from borate to the cyclometalated platinum moiety. Hence, the emission quenching upon fluoride binding is ascribed to PET process from Ar3B·F− moiety to Pt(OÔ)(C^N) moiety.

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CONCLUSIONS A new class of triarylborane-functionalized acetylacetonato Pt(II) complexes is synthesized, and their photophysical characteristics are explored. The complexes exhibit bright luminescence both in the solid and solution states with moderate quantum yields. A change of cyclometalating ligand (2-phenylpyridine, 2-thiophenylpyridine, and 2-thianapthenylpyridine) leads to a red shift in the emission from green to red. Lewis-acidic boron, molecular rigidity, as well as intermolecular interactions play a crucial role in determining the emission efficiency of the reported complexes. As the molecular rigidity increases due to the presence of the bulky duryl group, complexes 2, 4, and 6 exhibit higher luminescence quantum yields compared to 1, 3, and 5 in the solid state. These results show that the TAB-functionalized ancillary ligand can be used to enhance the luminescence from cyclometalated platinum complexes. The salient photophysical features of complexes 1− 6 make them useful for optoelectronic applications.

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EXPERIMENTAL SECTION

All reactions were performed under an atmosphere of purified nitrogen using standard Schlenck techniques. Acetonitrile and chlorinated solvents were distilled over CaH2 and subsequently degassed three times by freeze−pump−thaw method. The 400 MHz 1H NMR and 100 MHz 13C NMR spectra were recorded on a Bruker Advance 400 MHz NMR spectrometer. Electronic absorption spectra were recorded on a Perkin- Elmer LAMBDA 750 UV−vis spectrophotometer. Solutions were prepared using a microbalance (±0.1 mg) and volumetric glassware and then charged in quartz cuvettes with sealing screw caps. Photoluminescence emission studies were carried out on a Horiba JOBIN YVON Fluoromax-4 spectrometer. Single crystals of 2−4 were grown by slow evaporation of chloroform/toluene solutions of the respective compounds. The diffraction data of 2−4 were collected on a Bruker SMART APEX CCD diffractometer using the SMART/SAINT software.15 Intensity data were collected using graphite-monochromatic Mo Kα radiation (0.7107 Å) at 160(2) K on a crystal as obtained after several attempts. The structures were solved by WinGx software. High-resolution mass spectra were obtained on a JEOL SX-120/DA6000 spectrometer using argon (6 kV, 10 mA) as the FAB gas. Quantum yield measurement was carried out using a calibrated integrating sphere. Caution! Fluoride is extremely toxic anion and should be handled with extreme care and attention. Although very small quantities of fluoride salts were used in this study, thorough laboratory safety protocols were strictly followed while handling such compounds (tetrabutylammonium fluoride). Synthetic Procedures. Boryl acetylacetone ligands (A, B) were synthesized by previously reported literature methods.13 General Procedure for the Preparation of Pt (OÔ)(C^N) Complexes (1−6). A 50 mL two-necked round-bottom flask fitted with a reflux condenser was charged with predried degassed acetonitrile (25 mL), ligand A/B, and Na2CO3. The reaction mixture was stirred for 1 h, μ-chlorobridged cyclometalated platinum dimer was added, and the reaction mixture was heated under reflux for 36 h. Evaporation of the solvent gave crude metal complexes which were further purified by column chromatography over silica gel (hexane/ EtOAc 90:10) chromatography. Quantities of reactants involved and the characterization data are given below. Complex 1. A (0.115 g, 0.273 mmol), Na2CO3 (0.07 g, 0.645 mmol), platinum dimer (0.1 g, 0.129 mmol), acetonitrile (25 mL). Yield: 0.076 g (76%) of green color solid. 1H NMR (400 MHz, CDCl3, δ ppm) 8.99 (d, J = 4 Hz, 1H), 7.81 (t, J = 4 Hz, 1H), 7.64 (d, G


Article

Inorganic Chemistry H+] 885.3350. Anal. Calcd for C46H48BNO2PtS: C, 62.44; H, 5.47; N, 1.58. Found: C, 62.5; H, 5.27; N, 1.63.

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(b) Fillaut, J. L.; Akdas-Kilig, H.; Dean, E.; Latouche, C.; Boucekkine, A. Switching of reverse charge transfers for a rational design of an OFF−ON phosphorescent chemodosimeter of cyanide anions. Inorg. Chem. 2013, 52, 4890−4897. (c) Dorazco-Gonzalez, A. Chemosensing of chloride based on a luminescent platinum(II) NCN pincer complex in aqueous media. Organometallics 2014, 33, 868−875. (6) (a) Davies, G. M.; Pope, S. J. A.; Adams, H.; Faulkner, S.; Ward, M. D. Structural and photophysical properties of coordination networks combining [Ru(bipy) (CN)4]2‑ anions and lanthanide(III) cations: rates of photoinduced Ru-to-lanthanide energy transfer and sensitized near-infrared luminescence. Inorg. Chem. 2005, 44, 4656− 4665. (b) Shavaleev, N. M.; Accorsi, G.; Virgili, D.; Bell, Z. R.; Lazarides, T.; Calogero, G.; Armaroli, N.; Ward, M. D. Syntheses and crystal structures of dinuclear complexes containing d-Block and fBlock luminophores. sensitization of NIR luminescence from Yb(III), Nd(III), and Er(III) centers by energy transfer from Re(I)− and Pt(II)−bipyrimidine metal centers. Inorg. Chem. 2005, 44, 61−72. (c) Di Piazza, E.; Norel, L.; Costuas, K.; Bourdolle, A.; Maury, O.; Rigaut, S. d−f Heterobimetallic association between ytterbium and ruthenium carbon-rich complexes: redox commutation of near-IR luminescence. J. Am. Chem. Soc. 2011, 133, 6174−6176. (d) Li, X. L.; Tan, M.; Zhang, K. J.; Yang, B.; Chen, J.; Ai, Y. B. Dual luminescent tetranuclear organogold(I) macrocycles of 5,5′-diethynyl-2,2′-bipyridine and their efficient sensitization of Yb(III) luminescence. Inorg. Chem. 2012, 51, 109−118. (7) (a) Clem, T. A.; Kavulak, D. F. J.; Westling, E. J.; Frechet, J. M. J. Cyclometalated platinum polymers: synthesis, photophysical properties, and photovoltaic performance. Chem. Mater. 2010, 22, 1977− 1987. (b) Cheng, G.; Chow, P. K.; Kui, S. C. F.; Kwok, C. C.; Che, C. M. High-efficiency polymer light-emitting devices with robust phosphorescent platinum(II) emitters containing tetradentate dianionic O^N^C^N ligands. Adv. Mater. 2013, 25, 6765−6770. (c) Krikorian, M.; Liu, S.; Swager, T. M. Columnar liquid crystallinity and mechanochromism in cationic platinum(II) complexes. J. Am. Chem. Soc. 2014, 136, 2952−2955. (d) Zhang, Y.; Clavadetscher, J.; Bachmann, M.; Blacque, O.; Venkatesan, K. Tuning the luminescent properties of Pt(II) acetylide complexes through varying the electronic properties of N-heterocyclic carbene ligands. Inorg. Chem. 2014, 53, 756−771. (e) Sivchik, V. V.; Grachova, E. V.; Melnikov, A. S.; Smirnov, S. N.; Yu, A.; Ivanov; Hirva, P.; Tunik, S. P.; Koshevoy, I. O. Solid-state and solution metallophilic aggregation of a cationic [Pt(NCN)L]+ cyclometalated complex. Inorg. Chem. 2016, 55, 3351−3363. (8) Balzani, V.; Campagna, S. Photochemistry and photophysics of coordination compounds II; Springer: Heidelberg, 2007. (9) (a) Lu, W.; Mi, B. X.; Chan, M. C. W.; Hui, Z.; Che, C. M.; Zhu, N.; Lee, S. T. Light-emitting tridentate cyclometalated platinum(II) complexes containing σ-alkynyl auxiliaries: tuning of photo- and electrophosphorescence. J. Am. Chem. Soc. 2004, 126, 4958−4971. (b) Jenkins, D. M.; Bernhard, S. Synthesis and characterization of luminescent bis-cyclometalated platinum IV complexes. Inorg. Chem. 2010, 49, 11297−11308. (c) Tronnier, A.; Risler, A.; Langer, N.; Wagenblast, G.; Münster, I.; Strassner, T. A phosphorescent C^C* cyclometalated platinum(II) dibenzothiophene NHC complex. Organometallics 2012, 31, 7447−7452. (d) Bossi, A.; Rausch, A. F.; Leitl, M. J.; Czerwieniec, R.; Whited, M. T.; Djurovich, P. I.; Yersin, H.; Thompson, M. E. Photophysical properties of cyclometalated Pt(II) complexes: counterintuitive blue shift in emission with an expanded ligand π system. Inorg. Chem. 2013, 52, 12403−12415. (e) Jana, A.; McKenzie, L.; Wragg, A. B.; Ishida, M.; Hill, J. P.; Weinstein, J. A.; Baggaley, E.; Ward, M. D. Porphyrin/Platinum(II) C^N^N acetylide complexes: synthesis, photophysical properties, and singlet oxygen generation. Chem. - Eur. J. 2016, 22, 4164−4174. (10) (a) Sun, W.; Zhu, H.; Barron, P. M. Binuclear Cyclometalated platinum(II) 4,6-diphenyl-2,2′-bipyridine complexes: interesting photoluminescent and optical limiting materials. Chem. Mater. 2006, 18, 2602−2610. (b) Kui, S. C. F.; Chui, S. S. Y.; Che, C. M.; Zhu, N. Structures, photoluminescence, and reversible vapoluminescence properties of neutral platinum(II) complexes containing extended π-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01827. Crystallographic information, DFT computational data, and excitation spectra (PDF) (CIF) (CIF) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Pakkirisamy Thilagar: 0000-0001-9569-7733 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.T. thanks the cience and Engineering Research Board (SERB), Government of India and IISc, Bangalore, for the financial support. G.R.K. thanks the UGC New Delhi and IISc, Bangalore for research fellowships.

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REFERENCES

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