Long-Lived π-Shape Platinum(II) Diimine Complexes Bearing 7

Feb 21, 2013 - Department of Chemistry and Biochemistry, North Dakota State .... Christopher McCleese , Yu Fang , Svetlana Kilina , Yinglin Song , Cle...
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Long-Lived π‑Shape Platinum(II) Diimine Complexes Bearing 7‑Benzothiazolylfluoren-2-yl Motif on the Bipyridine and Acetylide Ligands: Admixing π,π* and Charge-Transfer Configurations Yuhao Li, Rui Liu, Ekaterina Badaeva, Svetlana Kilina, and Wenfang Sun* Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States S Supporting Information *

ABSTRACT: Two π-shape platinum(II) diimine complexes (1 and 2) bearing 7-benzothiazolylfluoren-2-yl motif on the bipyridine and acetylide ligands were synthesized, and their photophysics and reverse saturable absorption were investigated systematically. Both complexes exhibit lowenergy, broad, and weak 1MLCT (metal-to-ligand charge transfer)/1LLCT (ligand-to-ligand charge transfer) transitions and intense high-energy bipyridine ligand or acetylide ligand localized 1π,π* transitions mixed with some 1MLCT/1LLCT/1ILCT (intraligand charge transfer) characters in their UV−vis absorption spectra. The emission of 1 and 2 includes a weak fluorescence band and a broad, structureless phosphorescence band emanating from the 3MLCT/3LLCT/3π,π* states for 1 and predominantly from 3π,π* state for 2. Insertion of CC bond between the bipyridine and fluorene components induces pronounced red-shifts of the absorption and emission bands because of the extended π-conjugation in the bipyridine ligand. Because of the admixture of the 3π,π* excited state with the charge-transfer excited states, both complexes possess extremely long-lived, broadband (visible to the near-IR region) triplet excited-state absorption, resulting in remarkably strong reverse saturable absorption at 532 nm for nanosecond laser pulses. Therefore, complexes 1 and 2 are excellent candidates for devices that require strong reverse saturable absorption.

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INTRODUCTION In recent years, the photophysics of platinum(II) diimine complexes with bis(arylacetylide) ligands has been extensively investigated, and their potential applications in organic lightemitting diodes (OLEDs),1 molecular probes,2−5 photochemical devices,6,7 triplet−triplet annihilation-based upconversion,8−10 nonlinear transmission,11,12 and so forth have been reported. Castellano et al. revealed that the triplet metal-toligand charge transfer (3MLCT) excited state and the intraligand 3π,π* excited state could be switched when altering the conjugated aromatic rings on the acetylide ligands or when varying solvent polarity,13−15 which was echoed by our previous study on Pt(II) bipyridine complexes bearing 2-(benzothiazol2′-yl)-9,9-diethyl-7-ethynylfluorene ligands11 and by Zhao and co-workers on Pt(II) bipyridine complexes with naphthalenediimide, coumarin, or rhodamine substituted acetylide ligands.8−10 It was found that when the π-conjugation of the acetylide ligand was extended, the contribution of acetylide ligand localized 3π,π* state to the lowest triplet excited state became more significant, which led to long-lived triplet excited state and which favored the applications of Pt(II) complexes in oxygen sensing, upconversion, and nonlinear transmission.8−11 It was reported by the Eisenberg and Schanze groups independently that the MLCT state energy could be adjusted by variation of electron-donating or withdrawing substituent on the bipyridine ligand.16−18 Schanze and co-workers also discovered that the photophysics of the Ir(III) and Re(I) © 2013 American Chemical Society

complexes with 5,5′-bis(oligo(aryleneethynylene))-2,2′-bipyridine ligand admixed the 3MLCT and oligomer ligand based 3 π,π* characters because of the close proximity of these two excited states.19−21 However, there has been no report on how the extended π-conjugation on the bipyridine ligand influences the natures of the lowest singlet and triplet excited states in Pt(II) complexes. Specifically, when both the bipyridine ligand and the acetylide ligands contain π-conjugated aromatic scaffolds, the photophysics of the complexes becomes complicated. The lowest singlet and triplet excited states could be MLCT/LLCT (ligand-to-ligand charge transfer) in nature, could have the acetylide ligand π,π* parentage, could be dominated by the substituted bipyridine ligand π,π* state, or could admix some or all of these excited states. To answer this question, we synthesized two Pt(II) complexes with 7(benzothiazol-2′-yl)-9,9-di(2-ethylhexyl)fluoren-2-yl motif attached on both the bipyridine and the acetylide ligands (1 and 2 in Scheme 1) and systematically investigated their photophysics and nonlinear absorption in solution via experiments and time-dependent density functional theory (TDDFT) calculations. To aid us in the understanding of the natures of the lowest excited state in 1 and 2, their photophysics are Received: December 21, 2012 Revised: February 19, 2013 Published: February 21, 2013 5908

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Scheme 1. (a) (1) BuLi, −78 °C, Ar, 1.5 h; (2) 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (ITDB), −78 °C, Ar, 1.5 h Then rt 12 h. (b) 5,5′-Dibromo-2,2′-bipyridine, Pd(PPh3)4, K2CO3, H2O, Toluene, Ar, Reflux, 48 h. (c) Pt(DMSO)2Cl2, CH3CN/CH2Cl2 (1:1.5, v/v), Ar, Reflux, 24 h. (d) 2-(Benzothiazol-2′-yl)-9,9-bis(2-ethylhexyl)-7-ethynylfluorene (3L), CH2Cl2, N,N-Diisopropylethylamine, CuI, Ar, Reflux, 24 h. (e) 5,5′-Dibromo-2,2′-bipyridine, Pd(PPh3)4, n-Propylamine, Ar, 70 °C, 3 Days

(because of the exceptionally large ratio of the excited-state to ground-state absorption cross section)11 compared to that from the complex without the terminal benzothiazolyl substitutent.22 Moreover, the Pt(II) diimine complex with the terminal benzothiazolyl substituent also possesses large two-photon absorption cross sections in the near-IR region.11 In complex 2, a carbon−carbon triple bond is inserted between the bipyridine and the fluorene motif to further extend the π-conjugation in the bipyridine ligand.

compared to those of their corresponding ligands (1L and 2L) and their respective Pt(II) chloride complexes (1a and 2a). Benzothiazole (BTZ) was selected as the end-capping substituent, and fluorene was chosen as the rigid linker because both are conjugated aromatic systems and are considered as good building blocks for nonlinear absorbing materials (reverse saturable absorbers or two-photon absorbers).22−26 It was discovered by our group earlier that because of the πconjugation and the electron-withdrawing nature of the BTZ substituent, the acetylide ligand based 1π,π* transition in a Pt(II) diimine complex bearing 2-(benzothiazol-2′-yl)-9,9diethyl-7-ethynylfluorene ligands is red-shifted and the triplet transient absorption from the 3π,π* state is broader and redshifted compared to those of the corresponding Pt(II) diimine complex without the terminal benzothiazolyl substituent.22 As a result, the Pt(II) diimine complex with the terminal benzothiazolyl substituent exhibits much stronger reverse saturable absorption at 532 nm for nanosecond laser pulses

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EXPERIMENTAL SECTION Synthesis and Characterizations. The chemical reagents used for synthesis and the HPLC (high performance liquid chromatography) grade solvents for photophysical studies were obtained from Aldrich or Alfa Aesar and were used as is unless otherwise noted. Precursors 2-(benzothiazol-2′-yl)-7-bromo9,9-bis(2-ethylhexyl)fluorene (BTZ-F8-Br),27 BTZ-F8-B,28 and acetylide ligand 2-(benzothiazol-2′-yl)-9,9-bis(2-ethylhex5909

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yl)-7-ethynylfluorene (3L)27 were synthesized according to the literature procedures with replacement of reagent ethyl bromide to 2-ethylhexyl bromide. Precursor 5,5′-dibromo2,2′-bipyridine was also prepared according to the literature procedure.29 Suzuki coupling reaction of BTZ-F8-B and 5,5′dibromo-2,2′-bipyridine yielded ligand 1L, while Sonogashira coupling reaction of 3L and 5,5′-dibromo-2,2′-bipyridine gave ligand 2L. Reaction of 1L and 2L with Pt(DMSO)2Cl2 afforded Pt(II) chloride complexes 1a and 2a. Substitution of Cl coligand by 3L under basic condition in the presence of CuI catalyst produced the desired complexes 1 and 2. All the intermediate compounds were characterized by 1H NMR. Ligand 1L and 2L were characterized by 1H NMR, highresolution mass spectrometry (HRMS), and elemental analyses. 1a, 2a, 1, and 2 were characterized by 1H NMR and elemental analyses. HRMS data were unable to be obtained for 1a, 2a, 1, and 2 because of the difficulty in ionization of these samples. A Varian Oxford-400 or Oxford-500 VNMR spectrometer was used to obtain the 1H NMR spectra, and a Bruker BioTOF III mass spectrometer was used to conduct the electrospray ionization (ESI) HRMS analyses. Elemental analyses were performed by NuMega Resonance Laboratories, Inc. in San Diego, California. 1L. Compound BTZ-F8-B (398 mg, 0.61 mmol), 5,5′dibromo-2,2′-bipyridine (90 mg, 0.29 mmol), and Pd(PPh3)4 (33 mg, 0.029 mmol) were added to 20 mL toluene. Then, 2 M K2CO3 aqueous solution (1.45 mL) was added. The mixture was heated to reflux under argon for 48 h. After the reaction, the solvent was reduced in vacuum, and the residue was extracted with CH2Cl2 and was washed with brine. The organic layer was combined and was dried over MgSO4. After removal of solvent, the crude product was purified by column chromatography (Al2O3, eluent used was hexane:ethyl acetate = 15:1 to 10:1 v/v) to afford 1L as an off-white powder (0.11 g, 32% yield). 1 H NMR (400 MHz, CDCl3) δ (ppm) 8.99 (d, J = 2.4 Hz, 1H), 8.56 (dd, J1 = 8.2 Hz, J2 = 2.4 Hz, 1H), 8.16−8.06 (m, 4H), 7.92−7.83 (m, 3H), 7.69−7.66 (m, 2H), 7.49 (t, J = 7.8 Hz, 1H), 7.37 (t, J = 7.6 Hz, 1H), 2.19−2.06 (m, 4H), 0.84−0.79 (m, 16H), 0.63−0.50 (m, 14H). ESI-HRMS: (M + H)+ calcd m/z = 1199.6947; found, m/z = 1199.6993. Anal. calcd for C82H94N4S2: C, 82.09; H, 7.90; N, 4.67. Found: C, 82.43; H, 8.23; N, 4.62. 1a. Pt(DMSO)2Cl2 (35 mg, 0.08 mmol) and 1L (100 mg, 0.08 mmol) were added to the mixed solvent of CH3CN (20 mL) and CH2Cl2 (30 mL). The reaction mixture was heated to reflux for 24 h. Then, the solvent was removed, and the residue was recrystallized with CH2Cl2/hexane to afford yellow solid (89 mg, yield: 73%). 1H NMR (400 MHz, CDCl3) δ (ppm) 10.21 (s, 1H), 8.35 (d, J = 7.2 Hz, 1H), 818−8.02 (m, 4H), 7.92−7.84 (m, 3H), 7.69−7.62 (m, 2H), 7.50 (t, J = 7.8 Hz, 1H), 7.39 (t, J = 7.6 Hz, 1H), 2.21−2.08 (m, 4H), 0.88−0.79 (m, 16H), 0.61−0.49 (m, 14H). Anal. calcd for C82H94Cl2N4PtS2·H2O: C, 66.38; H, 6.52; N, 3.78. Found: C, 65.99; H, 6.91; N, 3.75. 1. Complex 1a (125 mg, 0.085 mmol), ligand 3L (103 mg, 0.188 mmol), and CuI (3 mg) were added to degassed mixed solvent of dry CH2Cl2 (50 mL) and diisopropylamine (20 mL). The mixture was refluxed under argon for 24 h. After removal of the solvent, the crude product was purified by column chromatography (silica gel, eluent was hexane:CH2Cl2 = 2:1 to 1:5, v/v) twice and then was recrystallized with CH2Cl2/hexane to afford red powder (15 mg, yield: 7%). 1H NMR (500 MHz, CDCl3) δ (ppm) 10.46 (s, 1H), 8.38 (s, 1H), 8.10 (ddd, J1 =

18.3 Hz, J2 = 13.3 Hz, J3 = 7.3 Hz, 7H), 7.88 (dt, J1 = 16.3 Hz, J2 = 7.9 Hz, 5H), 7.76 (dd, J1 = 11.1 Hz, J2 = 6.7 Hz, 2H), 7.71−7.65 (m, 2H), 7.62 (d, J = 7.7 Hz, 1H), 7.48 (dd, J1 = 18.4 Hz, J2 = 7.7 Hz, 2H), 7.41−7.33 (m, 2H), 2.26−1.94 (m, 8H), 0.95−0.75 (m, 33H), 0.69−0.47 (m, 27H). Anal. calcd for C161H190Cl2N6PtS4·C6H14: C, 76.50; H, 7.67; N, 3.26. Found: C, 76.57; H, 7.88; N, 3.19. 2L. Compounds 5,5′-dibromo-2,2′-bipyridine (0.327 g, 1.04 mmol), 3L (1.26 g, 2.29 mmol), and Pd(PPh3)4 (0.138 g, 0.12 mmol) were added to 30 mL degassed n-propylamine at room temperature (rt). The reaction mixture was then heated to reflux for 72 h. After removal of solvent, the crude product was purified by column chromatography (Al2O3, eluent was hexane:CH2Cl2 = 2:1 to 0:1 v/v) and was recrystallized from acetone; 0.36 g pale yellow powder was afforded (yield: 29%). 1 H NMR (400 MHz, CDCl3) δ (ppm) 8.85 (s, 1H), 8.46 (d, J = 8.4 Hz, 1H), 8.15−8.01 (m, 3H), 7.99−7.97 (m, 1H), 7.91 (d, J = 8.4 Hz, 1H), 7.80 (d, J = 8.0 Hz, 1H), 7.76 (d, J = 7.6 Hz, 1H), 7.63−7.58 (m, 2H), 7.50 (t, J = 7.6 Hz, 1H), 7.38 (t, J = 7.6 Hz, 1H), 2.13−2.08 (m, 4H), 0.94−0.71 (m, 19H), 0.61− 0.50 (m, 11H). ESI-HRMS: (M + H)+ calcd m/z = 1247.6947; found m/z = 1247.6924. Anal. calcd for C86H94N4S2: C, 82.78; H, 7.59; N, 4.49. Found: C, 82.31; H, 7.93; N, 4.46. 2a. Pt(DMSO)2Cl2 (34 mg, 0.08 mmol) and 2L (100 mg, 0.08 mmol) were added to the mixed solvent of CH3CN (30 mL) and CH2Cl2 (25 mL). The reaction mixture was heated up to reflux for 12 h. The solvent was then removed, and the residue was recrystallized with CH2Cl2/hexane to afford yellow solid 60 mg (yield: 50%). 1H NMR (400 MHz, CDCl3) δ (ppm) 9.96 (s, 1H), 8.24−8.23 (m, 1H), 8.16−8.08 (m, 3H), 7.93 (t, J = 6.8 Hz, 2H), 7.81 (d, J = 8.0 Hz, 1H), 7.55−7.23 (m, 1H), 7.67−7.66 (m, 1H), 7.56 (d, J = 7.6 Hz, 1H), 7.50 (t, J = 7.6 Hz, 1H), 7.39 (t, J = 7.6 Hz, 1H), 2.17−2.04 (m, 4H), 0.92−0.69 (m, 20H), 0.60−0.51 (m, 10H). Anal. calcd for C86H94Cl2N4PtS2: C, 68.23; H, 6.26; N, 3.70. Found: C, 67.73; H, 6.58; N, 3.78. 2. Ligands 2a (200 mg, 0.13 mmol) and 3L (159 mg, 0.29 mmol) and catalyst CuI (3 mg) were dissolved in degassed dry CH2Cl2 (50 mL) and diisopropylamine (20 mL). The reaction mixture was refluxed under argon for 24 h. After removal of the solvent, the crude product was purified by column chromatography (silica gel, eluent was hexane:CH2Cl2 = 2:1 to 1:5, v/v) twice to afford dark red powder 25 mg (yield: 7.5%). 1H NMR (400 MHz, CDCl3) δ (ppm) 10.15 (s, 1H), 8.25−8.23 (m, 1H), 8.13−8.05 (m, 6H), 8.02 (d, J = 8.0 Hz, 1H), 7.90 (d, J = 7.6 Hz, 2H), 7.81−7.69 (m, 6H), 7.62−7.60 (m, 1H), 7.54− 7.46 (m, 3H), 7.39−7.33 (q, J = 7.8 Hz, 2H), 2.17−2.07 (m, 8H), 0.95−0.67 (m, 4H), 0.58−0.49 (m, 19H). Anal. calcd for C162H182N6PtS4·H2O: C, 76.71; H, 7.26; N, 3.29. Found: C, 76.25; H, 7.75; N, 3.18. Photophysical Study. A Shimadzu UV-2501 spectrophotometer was used to measure the UV−vis absorption spectra of 1L, 2L, 1a, 2a, 1, and 2 in different solvents at room temperature; a SPEX Fluorolog-3 fluorometer/phosphorometer was used to acquire the steady-state emission spectra. To determine the emission quantum yields of 1L, 2L, 1a, 2a, 1, and 2 in degassed solutions, the relative actinometry30 method was applied. A degassed aqueous solution of [Ru(bpy)3]Cl2 (Φem = 0.042, λex = 436 nm)31 was utilized as the reference for complexes 1a, 2a, 1, and 2, and a degassed 1 N sulfuric acid solution of quinine bisulfate (Φem = 0.546, λex = 347.5 nm)32 was used as the reference for ligands 1L and 2L. An Edinburgh LP920 laser flash photolysis spectrometer with 355 nm 5910

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electron transition orbital, and we refer to the occupied NTO as the hole transition orbital. Note that electron and hole NTOs are not the same as unoccupied and occupied molecular orbitals (MOs) in their ground state. Upon photoexcitation, the excitonic couplings (Coulomb interaction between the photoexcited electron−hole pair) mix the ground-state MOs so that the representation of an excitation via the pair of ground-state MOs is not valid. In contrast, electron and hole NTOs obtained from TDDFT calculations allow for representation of the excited-state electronic density. NTOs shown in this paper were produced with the isovalue of 0.02 and were visualized with the GaussView 5.1 graphical software.48 It is necessary to point out that while overall absorption experimental trends are reproduced by the theoretical results, the calculated lower energy MLCT and the bipyridine-based π,π* transition energies are slightly underestimated within the calculation scheme described above. The main reasons for that are, first, the idealized solvent model used in the calculations and, second, underestimated torsional disorder of the bpy ligands, which in experiment (room temperature) is higher compared to the calculation (where geometry is optimized at 0 K). The considered ligand molecules are relatively flexible in terms of torsion angles. Therefore, the calculated equilibrium geometries are expected to be more planar than those in experimental samples taken at room temperature. Idealized planar structure of ligands for both complexes results in a small red-shift of the calculated excitation energies as compared to experimental spectra.

excitation from a Nd:YAG laser (Quantel Brilliant, pulsewidth ∼ 4.1 ns, 1 Hz) was used to determine the nanosecond transient difference absorption spectra, triplet excited-state lifetimes, and triplet excited-state quantum yields. Prior to each measurement, all the sample solutions were degassed with Ar for 30 min. Singlet depletion method33 was used to determine the triplet excited-state molar extinction coefficients (εT) at the transient absorption (TA) band maximum. The details of this method are described in the literature.33 After obtaining the εT value, the relative actinometry method34 was applied to measure the triplet excited-state quantum yield using SiNc in benzene as the reference (ε590 = 70 000 M−1 cm−1, ΦT = 0.20).35 Nonlinear Transmission Measurement. The nonlinear transmission experiments for complexes 1 and 2 at 532 nm were conducted in CH2Cl2 solutions in a 2 mm cuvette using 4.1 ns laser pulses. The light source used was the secondharmonic output from a Quantel Brilliant nanosecond laser with a repetition rate of 10 Hz. The experimental setup and details were described previously.36 An f = 30 cm plano-convex lens was used to focus the beam to the sample cuvette. The linear transmission of the solution was adjusted to 80% in the 2 mm cuvette. The radius of the beam waist at the focal plane was 96 μm. Quantum Chemistry Calculations. The ground- and excited-state properties of complexes 1 and 2 were studied using density functional theory (DFT) and linear response time dependent DFT (TDDFT). All calculations to characterize excited states, including the ground-state geometry optimization, electronic structure, optical spectra, and natural transition orbitals (NTOs), were performed using Gaussian09 quantum chemistry software package.37 Geometries of the complexes were optimized for the ground state (closed-shell singlet S0). For calculations of emission spectra, the geometry was optimized in the lowest-energy excited singlet (S1) and triplet (T1) states. All procedures were done utilizing the hybrid PBE1 functional.38,39 Pt ions were described by the Hay-Wadt relativistic effective core potential and the associated LANL08 basis set,40,41 while the remaining atoms were modeled with the 6-31G* basis set. Other functionals, such as hybrid B3LYP42 and long-range-corrected CAM-B3LYP,43 were also tested. While all functionals predict very similar characters of absorption and emission spectra, the PBE1 provides the best agreement with experimental spectra for complexes 1 and 2 studied in this work. All calculations have been performed in solvent using Conductor Polarized Continuum Model (CPCM)43,44 as implemented in Gaussian09. Dichloromethane (CH2Cl2, εr = 8.93) was chosen as a solvent for consistency with the experimental studies. For the absorption spectra of the complexes, 40 lowest singlet optical transitions were considered. The fluorescence (S*) and phosphorescence (T*) energies were determined by calculating vertical transition energies for the optimized lowest singlet (S1*) or optimized lowest triplet (T1*) excited state geometries, respectively, within the TDDFT formalism implementing optimization of the excited state.45,46 To analyze the nature of the singlet and triplet excited states, natural transition orbital (NTO) analysis was performed on the basis of the calculated transition densities.47 This method offers the most compact representation of the transition density between the ground and excited states in terms of an expansion into single-particle transitions (hole and electron for each given excited state). Here, we refer to the unoccupied NTO as the

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RESULTS AND DISCUSSION UV−Vis Absorption. The UV−vis absorption of 1L, 2L, 1a, 2a, 1, and 2 was studied in CH2Cl2 solutions with different concentrations (1 × 10−6 to 5 × 10−4 mol/L). It is found that Beer’s law is followed by all of the compounds in the concentration range studied, suggesting the absence of groundstate aggregation in this concentration range for these compounds. This can be attributed to the branched alkyl chains on the fluorene motif, which prevent the intermolecular interactions. The absorption spectra of 1L and 2L (Figure 1a) are characterized by the long-axis-polarized 1π,π* transitions, while the spectra of their Pt(II) chloride complexes 1a and 2a (Figure 1b) feature two major bands. The low-energy bands at ca. 410 nm for 1a and 440 nm for 2a could be attributed to the long-axis-polarized 1π,π* transition mixed with the intraligand charge transfer (1ILCT, π(BTZ-fluorene)→π*(bpy)) transition, while the shoulder at ca. 450 nm for 1a and 470 nm for 2a should predominantly arise from the dπ(Pt)→π*(bpy) 1MLCT transition overlapping with the 1π,π* and 1ILCT transitions. The 1MLCT nature of this shoulder can be supported by the comparison of the UV−vis absorption spectra of 1a and 2a to those of their respective Zn(II) complex counterparts in which this shoulder is absent (see Figure S1 of the Supporting Information). The red-shift of the major absorption bands at ca. 410 and 440 nm for 1a and 2a, respectively, with respect to those in 1L and 2L should be attributed to the stabilization of the ligand-based lowest unoccupied molecular orbital (LUMO) after complexation with the Pt(II) ion, which is similar to that observed in an Ir(III) complex bearing phenyleneethynylene substituted bipyridine ligand.21 In addition, the involvement of the 1ILCT transitions could also account for this red-shift. For complexes 1 and 2, the UV−vis absorption spectra are featured by several major bands (Figure 1c and Table 1). By comparison of these bands to those in 1a, 2a, 1L, 2L, and 3L as 5911

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transfer bands in 1 and 2 with respect to those in 1a and 2a, which is attributed to the increased energy of Pt/acetylide based HOMOs in 1 and 2. Insertion of the CC bond between the fluorene and bipyridine motifs in 2L, 2a, and 2 not only extends the conjugation length of the bpy-derived ligand, which causes the red-shifted absorption bands in 2L, 2a, and 2 with respect to those in 1L, 1a, and 1, but also allows for a more planar geometry for the bpy-derived ligand in 2 compared to the corresponding ligand in 1 (supported by DFT geometry optimization, see discussion below and Figure S4 of the Supporting Information). Such modification leads to stronger delocalization and results in stabilization of the bpy-derived 1 π,π* excited state that gives rise to a significant shoulder around 450 nm. As confirmed by DFT calculations, insertion of the CC bond between the fluorene and the bipyridine components results in a more planar geometry of the substituted bipyridine ligand in 2 relative to those in 1 (see Figure S4 of the Supporting Information). The calculated torsional angle between the fluorene and the bipyridine planes in 1 is ∼36°; this angle is noticeably reduced to ∼7° in 2. An additional effect is observed in the torsion of the acetylide ligands with respect to the main Pt-complex plane. The corresponding torsional angle in 1 is ∼56°, which decreases to ∼42° in 2. The Cartesian coordinates of the optimized complex geometries are presented in Tables S1 and S2 of the Supporting Information. In their excited states, both complexes demonstrate slight planarization of the substituted bipyridine ligand. The torsional angles between the fluorene motif in the bipyridine ligand and the bipyridine plane are reduced to ∼29° in 1 and to ∼5° in 2. A stronger decrease in torsional angles between the acetylide ligands and the Pt-coordination plane is observed; the angles are reduced to 44° and 29° in 1 and 2, respectively. Emission. The emission of 1L, 2L, 1a, 2a, 1, and 2 was investigated in different solvents at room temperature and in butyronitrile glassy matrix at 77 K (Figure 1, Figure 2, and Figures S5−S11 of the Supporting Information). The emission quantum yields and lifetimes are provided in Table 1 and Table S3 of the Supporting Information. The structured emission from 1L and 2L is mirror-image to the 1π,π* absorption band and is short-lived (700 nm) was observed by Schanze et al. in an Ir(III) complex bearing 5,5′bis(phenyleneethynylene) bipyridine ligand.21 The different origins of the fluorescence and phosphorescence bands are supported by their different excitation spectra when monitored at the fluorescence and phosphorescence bands, respectively (see Figure S5 of the Supporting Information), and by their

Figure 2. Normalized emission spectrum of 1 in different degassed solvents at room temperature (λex = 436 nm) and in butyronitrile glassy matrix at 77 K.

this emission band to that of their corresponding Zn(II) complex counterpart in which the emission is 1ILCT in nature in CH2Cl2 solution (see Figure S12 of the Supporting Information). The 1ILCT/1MLCT fluorescence has also been observed by McMillin et al. in a Pt(II) 4′-pyrenylterpyridyl chloride complex (Pt(4′-Pyre1-T)Cl+).49 In contrast, the phosphorescence features a vibronic spacing of 1414 cm−1 for 5913

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thermally induced Stokes shifts for 2 is consistent with the π,π* nature of the emitting state. Transient Absorption. To further understand the triplet excited-state character and kinetics, nanosecond time-resolved transient absorption (TA) of 1L, 2L, 1a, 2a, 1, and 2 was investigated in toluene (Figure 3). The measurement was done

different sensitivities to oxygen quenching. As exemplified in Figure S6 of the Supporting Information for 2a, the high-energy fluorescence band is less sensitive to oxygen quenching, while the low-energy phosphorescence bands are more sensitive to the presence of oxygen. Another piece of evidence supporting the assignment of the phosphorescence band of 1a and 2a to the bipyridine ligand localized 3π,π* excited state is the small thermally induced Stokes shift. As demonstrated in Figure S7 of the Supporting Information for the emission spectrum of 2a in BuCN solution at room temperature and at 77 K, the thermally induced Stokes shift is only 156 cm−1, which is typical for the 3 π,π* based emission. Although dual emission in Pt(II) complexes is uncommon, it is not unprecedented for organometallic complexes containing heavy transition metals, such as Ru(II) and Os(II) complexes.51−53 Recently, dual emission in Pt(II) acetylide complexes with carbazole-grafting 2-(2-pyridyl)benzimidazole ligand54 or 6-phenyl-4-(9,9-dihexylfluoren-2-yl)-2,2′-bipyridine ligand,55 in the mononuclear Pt(II) 4-pyrenylterpyridyl complex (Pt(4′-Pyre1-T)Cl+),49 in a back-to-back dinuclear Pt(II) terpyridyl complex,56 and in Pt(II) porphyrin−fluorene copolymers has also been reported.57 Moreover, the dual emission in 1a and 2a and in 1 and 2 (which will be discussed in the following paragraph) should not be surprising because as the π-conjugation of the bipyridine ligand increases, the spin− orbit coupling in the Pt(II) complex decreases because of reduced contribution of Pt-centered orbitals to the frontier molecular orbitals.58 Consequently, the intersystem crossing in these complexes is not efficient, which is verified by the very small triplet quantum yields (listed in Table 1) in these complexes that are determined by the transient absorption measurement. For complexes 1 and 2, when excited at 425 nm for 1 and at 430 nm for 2 in CH2Cl2 solutions, the emission is dominated by phosphorescence at ca. 610 nm (τem = 450 ns) for 1 and 660 nm for 2 with a weak structureless fluorescence at ca. 450 and 490 nm for 1 and 2, respectively (see Figure S8 of the Supporting Information). The excitation spectra of 1 and 2 monitored at the respective fluorescence and phosphorescence bands are given in Figure S9 of the Supporting Information. TDDFT calculation results suggest that the fluorescence emanates from the 1MLCT/1LLCT states (Figure S10 of the Supporting Information). Unfortunately, the lifetime of the fluorescence was unable to be measured because of the weak fluorescence signal. The phosphorescence band of 1 and 2 exhibits a different trend in response to the solvent polarity variation. The phosphorescence band of 1 displays a pronounced negative solvatochromic effect (Figure 2), suggesting a less polar triplet excited state than the ground state, while the phosphorescence energy of 2 is less sensitive to the polarity of the solvent (Figure S11 of the Supporting Information). This suggests that the nature of the tripletemitting state in 1 and 2 could be different. TDDFT calculation confirms that the triplet-emitting state in 1 is predominantly 3 MLCT/3LLCT mixed with some acetylide ligand based 3π,π* character, while the origin of phosphorescence in 2 is dominated by the bipyridine ligand localized 3π,π* state with a small portion of 3MLCT character (Figure S10 of the Supporting Information). The different origins of the phosphorescence of 1 and 2 are further supported by the different thermally induced Stokes shifts, which are approximately 807 and 560 cm−1 for 1 and 2, respectively. The small

3

Figure 3. (a) The triplet transient difference absorption spectra of 1L, 2L at zero time delay in toluene. (b) The triplet transient difference absorption spectra of 1a, 2a at zero time delay in toluene. (c) Timeresolved triplet transient difference absorption spectra of 1 in toluene. (d) Time-resolved triplet transient difference absorption spectra of 2 in toluene. λex = 355 nm. The concentration of the solution was adjusted to obtain A = 0.4 at 355 nm in a 1 cm cuvette.

in toluene rather than in CH2Cl2 because the toluene solutions are more stable than CH2Cl2 solutions upon several hundreds of laser excitation at 355 nm during the TA measurement although the CH2Cl2 solutions are stable enough for the UV− vis and emission measurements. Ligands 1L and 2L exhibit very broad, moderately strong absorption from 420 to 770 nm, and bleaching occurs at ∼390 nm. The excited-state lifetimes deduced from the decay of the TA are 48.7 and 66.3 μs for 1L and 2L, respectively. The long lifetime suggests that the excited state that gives rise to the TA should be the 3π,π* state. The TA features of 1a and 2a are quite similar to those of 1L and 2L with bleaching appearing at approximately 420 nm and with broad absorption bands in the visible to the near-IR region. The position of the bleaching band is consistent with the 1 π,π*/1MLCT/1ILCT bands in their respective UV−vis absorption spectra. This feature along with the long lifetime and similar shape of the TA spectra to those of 1L and 2L suggests that the observed TA for 1a and 2a is predominantly ligand-localized 3π,π* in nature likely mixed with some 3 MLCT/3ILCT characters. This assignment is partially supported by the red-shift of the TA band maximum in 2a (535 nm) with respect to that in 1a (500 nm). The TA features of 1 and 2 are similar to those of 1a and 2a with the positive band maximum appearing at 537 nm for 1 and at 525 nm for 2, which is obviously red-shifted for 1 compared to that of 1a. Bleaching occurs at ∼390 nm for both 1 and 2. The triplet lifetimes deduced from the decay of TA for 1 and 2 are 52.6 and 47.1 μs, respectively, which are in line with those obtained from 1L, 2L, 1a, and 2a. Taking these facts into consideration, along with the nature of the lowest triplet excited state predicted by the TDDFT calculations (Figure S10), we tentatively assign the TA of 1 arising from the acetylide ligand localized 3π,π* state with significant mixing from the 3 MLCT/3LLCT states. For 2, the TA seems to predominantly 5914

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originate from the bipyridine ligand based 3π,π* state. The triplet excited-state absorption molar extinction coefficients at the TA band maximum and the triplet quantum yields were obtained using the singlet depletion method33 and the relative actinometry,34 respectively, and the results are listed in Table 1. The triplet lifetimes deduced from the decay of TA for 1L, 1a, and 2a are quite similar, which appears to be inconsistent with the heavy atom effect. This phenomenon can be rationalized by the reduced spin−orbit coupling when the πconjugation of the ligand increases as reported by Dubinina et al. for phenylene vinylene platinum(II) acetylides.58 The same reason accounts for the small triplet quantum yields in 1a, 2a, 1, and 2. Although the heavy-atom effect is not apparent in 1a, 2a, 1, and 2, coordination of the Pt(II) metal to the ligands 1L and 2L admixes ILCT and MLCT characters to the lowest excited states. This causes red-shifts of the lowest-energy absorption bands in 1a, 2a, 1, and 2, making it possible to populate the excited-state absorption via one-photon absorption in the visible spectral region (which is an essential requirement for reverse saturable absorbers) and allowing for reverse saturable absorption to occur. Reverse Saturable Absorption. Because complexes 1 and 2 exhibit weak ground-state absorption above 500 nm while the triplet excited-state absorption is stronger from 430 to 620 nm, reverse saturable absorption (RSA, i.e., transmission decreases with increased incident energy) is anticipated to occur in the spectral region between 500 and 620 nm. To demonstrate this, nonlinear transmission experiments were conducted for 1 and 2 in CH2Cl2 solution in a 2 mm cuvette using 4.1 ns, 532 nm laser pulses. To evaluate the effect of 5,5′-bis(7-(benzothiazol2′-yl)-9,9-di(2-ethylhexyl)fluoren-2-yl) substitution at the bipyridine ligand on the RSA of the Pt(II) bipyridine complexes, the RSA of the Pt(II) complexes with the same acetylide ligands but without the benzothiazolylfluorenyl motif on the bipyridine ligand (complex 1 in ref 11) was also studied under the same experimental condition. The transmission versus incident energy curves for 1, 2, and the reference complex are depicted in Figure 4. When the incident energy increases, the transmission of all three complexes decreases remarkably, suggesting a very strong RSA. The RSA of 1 and 2 are pronouncedly stronger than that of the reference Pt

complex in ref 11 with the RSA of 1 being slightly stronger than that of 2. To explain the exceptional RSA of 1 and 2, the ratios of the effective triplet excited-state absorption cross section to that of the ground state were estimated on the basis of the triplet TA spectra at zero delay, the UV−vis absorption spectrum, and the equation ΔOD(λ) = εT(λ)cTl − ε0(λ)cl (where εT and ε0 are the respective molar extinction coefficients of the triplet excited state and the ground state at the interested wavelength λ, cT and c are the respective molar concentration of the molecule at the triplet excited state and at the ground state, and l is the path length of the cuvette). The ε0(λ)cl can be deduced from the ground-state absorption spectrum of the same sample solution used for the TA measurement, which equals the absorbance of the solution; the ΔOD(λ) term can be obtained from the TA spectrum; the εT at the band maximum can be measured using the singlet depletion method;33 and thus, cT can be calculated from the aforementioned equation. Applying the ΔOD, ε0(λ)cl, and cT at different wavelengths to the equation, the εT at different wavelengths can be estimated. Conversion of ε to cross section (σ/cm2) can be conducted according to the equation σ = 3.82 × 10−21ε. The resultant values at 532 nm for 1 and 2 are listed in Table 3. These σT values are much larger than the cross sections reported in the literature for other reverse saturable absorbers.28,36,55,59−76 It is quite apparent that the σ0 values of 1 at each of the wavelengths are smaller than those of 2, while the σT values of 1 are larger than those of 2, which results in larger ratios of σT/σ0 for 1 than for 2 and leads to stronger RSA in 1. Although the estimated σT/σ0 values at 532 nm are smaller for 1 and 2 than that for the reference Pt(II) complex reported by our group previously,11,12 the much longer lifetimes of 1 and 2 could compensate for the smaller σT/σ0 values and could enhance the reverse saturable absorption. On the other hand, the σT values obtained by the aforementioned method are just a rough estimate of the bottom line for the σT values. The accurate σT values will be determined by the Z-scan measurement and by the fitting of the Z-scan data using a five-level model as reported by our group earlier.11,12 Nonetheless, to the best of our knowledge, the observed RSA of 1 and 2 is the strongest at 532 nm for nanosecond laser pulses compared to the known reverse saturable absorbers reported to date.

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CONCLUSION Two new π-type Pt complexes were synthesized, and their photophysics and reverse saturable absorption were investigated. The lowest singlet excited state of both complexes is 1 MLCT/1LLCT in nature, while the high-energy bands are dominated by the bipyridine ligand based and acetylide ligand localized 1 π,π* transitions with some mixture of 1 MLCT/1LLCT/1ILCT characters. Both complexes exhibit dual emission with the weak fluorescence band possibly originating from the 1MLCT/1LLCT states, while the phosphorescence emanates from the 3MLCT/3LLCT/3π,π* for 1 and predominantly from 3π,π* for 2. Introducing CC bond between the bipyridine and fluorene motifs causes a pronounced red-shift of the absorption and emission bands because of the extension of the π-conjugation in the bipyridine ligand. Both complexes possess relatively broad triplet excitedstate absorption in the visible to the near-IR region, which is tentatively assigned to the acetylide ligand localized 3π,π* state

Figure 4. Transmission vs incident energy curve for 1, 2, and the reference Pt(II) complex (complex 1 in ref 11) in CH2Cl2 solution at 532 nm using 4.1 ns laser pulses. The linear transmission of the solution is 80% in a 2 mm cuvette. The radius of the beam waist at the focal plane was 96 μm. 5915

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Table 3. Ground-State and Excited-State Absorption Cross Sections of 1 and 2 at Different Wavelengths σ0a/10−18 cm2

a

σTb/10−18 cm2

σT/σ0

λ/nm

1

2

1

2

1

2

450 475 500 532 550 575 600 625 650

99.8 39.8 22.0 7.60 3.23 0.876 0.241 0.00956 0.00650

67.3 17.2 9.32 5.93 2.66 0.861 0.249 0.00574

479 732 860 1200 1120 782 526 376 336

644 691 764 723 397 349 318 77

4.8 18 39 158 348 894 2.18 × 103 3.93 × 104 5.17 × 104

9.6 40 82 122 149 405 1.28 × 103 1.34 × 104

Ground-state absorption cross section in CH2Cl2, σ0 = 3.82 × 10−21ε0. bEffective triplet excited-state absorption cross section in toluene.

significantly mixed with the 3MLCT/3LLCT characters for 1 but to the bipyridine ligand based 3π,π* state for 2. The most striking feature of these two complexes is their very long triplet excited state lifetimes, which leads to remarkably strong reverse saturable absorption of these two complexes at 532 nm for nanosecond laser pulses. Compared to the corresponding Pt(II) bipyridine complex previously reported by our group in ref 11, incorporation of the benzothiazolylfluorenyl motif to the bipyridine ligand dramatically increases the lifetime of the absorbing triplet excited state, which makes the Pt(II) complexes 1 and 2 among the strongest reverse saturable absorbers at 532 nm for nanosecond laser pulses, and they are excellent candidates for devices that require strong reverse saturable absorption.

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ASSOCIATED CONTENT

S Supporting Information *

TDDFT calculation results for emission of 1 and 2; solventdependency UV−vis and emission spectra and emission data for 1 and 2; excitation spectra of 1a, 2a, 1, and 2; and full author list for refs 37 and 72. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 701-231-6254. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is partially supported by the National Science Foundation (CAREER CHE-0449598) and is partially supported by the Army Research Laboratory (W911NF-10-20055).

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REFERENCES

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dx.doi.org/10.1021/jp312642g | J. Phys. Chem. C 2013, 117, 5908−5918