Platinum(II) acetylide complexes with star- and V-shaped configurations possessing good trade-off between optical transparency and optical power limiting performance

C. Yao a, Z. Tian a, D. Jin a, F. Zhao a, Y. Sun a, X. Yang a, G. Zhou *a and W.-Y. Wong *bc
aMOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Institute of Chemistry for New Energy Material, Department of Chemistry, Faculty of Science, Xi’an Jiaotong University, Xi’an 710049, P. R. China. E-mail: zhougj@mail.xjtu.edu.cn; Fax: +86-29-8266-3914
bDepartment of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P. R. China. E-mail: wai-yeung.wong@polyu.edu.hk
cHong Kong Baptist University Institute of Research and Continuing Education, Shenzhen Virtual University Park, Shenzhen 518057, P. R. China. E-mail: rwywong@hkbu.edu.hk

Received 6th August 2017 , Accepted 9th October 2017

First published on 10th October 2017


By employing the central ligand with the desired structures, two series of Pt(II) acetylide complexes containing dimesitylborane (–B(Mes)2) and phenyl units as the terminal groups with star- and V-shaped configurations have been synthesized. Photophysical investigations have indicated that the –B(Mes)2 group can enhance triplet (T1) emission of the concerned Pt(II) acetylide molecules by enhancing the mixing of molecular orbitals from organic ligands and Pt(II) centers. The much higher T1 emission intensity associated with TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B can render them much stronger optical power limiting (OPL) response than their counterparts, TEB-Pt-Ph, CAZ-Pt-Ph and TPA-Pt-Ph respectively, terminated with phenyl groups. In addition, OPL ability of TPA-Pt-B for 532 nm nanosecond laser pulse can even surpass that of state-of-the-art OPL material C60. In addition, most Pt(II) acetylide complexes show OPL figure of merit factor σex/σo higher than that of C60. Importantly, the star-shaped molecular configuration employed by the Pt(II) acetylide units has the advantage of introducing more Pt(II) centers to the molecular skeleton while maintaining optical transparency of the concerned Pt(II) acetylide complexes. So, this research can open a new outlet for fulfilling the optimized trade-off between OPL activity and optical transparency for OPL materials based on reverse saturable absorption (RSA) mechanism.


Introduction

Optical power limiting (OPL) behavior represents a nonlinear optical effect that can clamp the energy of a laser beam after the incident irradiance reaches a certain threshold to make output fluence nearly stay at a particular level by further increasing input fluence.1–3 Clearly, OPL effect based on nonlinear optics (NLO) can be employed for protecting human eyes, optical sensors and sensitive optical components from the possible damage of sudden exposure to an intense laser beam.4–15 Compared with other laser-protecting strategies based on linear optics, such as absorption, reflection and diffraction, OPL effect can show inherent advantages of high transmittance, less energy loss at the safe laser power level and low vision loss, which makes it very suitable for practical application.16–18 Hence, with the wide application of laser, it is very desirable and urgent to develop OPL materials for protection. At present, both inorganic and organic materials have been successfully developed with good OPL properties.19,20 With the advantages of fast response speed (in the picosecond regime), large optical nonlinearity, versatile tuning of properties and easy processing, organic OPL materials have drawn increasing attention as compared to their inorganic counterparts.4,21 Unfortunately, the strong absorption bands in the visible-light region associated with high-performance OPL materials including fullerenes (e.g., C60), metal phthalocyanines and metalloporphyrins have restrained their application in practical devices for the protection of human eyes.22 It means that traditional organic OPL materials cannot achieve the trade-off between high OPL activity and good optical transparency, which represents a key problem in the field of NLO.

Recently, it has been shown that acetylide complexes of Pt(II), Hg(II), Au(I) and Pd(II) can serve as high-performance OPL materials.4,23,24 More importantly, Pt(II) acetylide molecules bearing phosphine ligands exhibit exceptional optical transparency in the visible-light region (ca. 400–700 nm) while maintaining more enhanced OPL ability than C60, Cu(II)–phthalocyanines and Zn(II)–porphyrins.4,22 In particular, the optical transparency of these Pt(II) acetylides can be further improved by both optimizing the configuration of Pt(II) centers and copolymerizing the backbone with Hg(II) ions.22,25 It has been shown that Pt(II) acetylides successfully fulfill the excellent trade-off between high OPL activity and good optical transparency, which can overcome the drawbacks of traditional OPL materials. Furthermore, Pt(II) acetylide molecules have been employed to make prototype OPL devices.16–18 All of these encouraging results have indicated that the Pt(II) acetylide materials can be a new generation of OPL materials with excellent optical transparency.4

The success of Pt(II) acetylides as OPL materials can be attributed to the electronic properties of organic acetylene ligands as well as their interaction with metal centers. Obviously, changing the structure of organic acetylene ligands can definitely exert a great influence on the OPL behaviors of Pt(II) acetylides.23,24,26 Typically, the reported Pt(II) acetylides for OPL investigation adopt linear configuration for both small molecules and polymers.23–26 What would be the OPL properties of Pt(II) acetylides with other configurations? Bearing this in mind, a series of Pt(II) acetylides with star- and V-shaped configurations have been prepared by employing central organic acetylene ligands with different structures. In addition, the electronic features for both central and terminal organic acetylene ligands have been altered to tune their OPL properties. So, the concerned results will provide valuable structure-OPL property information of Pt(II) acetylides for developing new high-performance OPL materials.

Experimental

General information

The commercially available reagents were used directly without further purification. All reactions were performed under an inert atmosphere. The solvents were purified by standard methods under dry nitrogen prior to use. The reactions were monitored by thin-layer chromatography (TLC) with Merck pre-coated aluminum plates. Flash column chromatography and preparative TLC were carried out using silica gel. All Sonogashira reactions were carried out with Schlenk techniques under nitrogen atmosphere.

Physical characterization

1H-, 13C- and 31P-NMR spectra were measured in CDCl3 with a Bruker AXS 400 MHz NMR spectrometer with 1H- and 13C-NMR chemical shifts quoted relative to SiMe4 and 31P chemical shifts relative to the 85% H3PO4 external standard. UV-vis absorption spectra were recorded with a Shimadzu UV-2250 spectrophotometer. The photoluminescent (PL) properties of the Pt(II) acetylides were characterized with an Edinburgh Instruments FLS920 fluorescence spectrophotometer. The low-temperature PL spectra and lifetimes at 77 K were obtained by dipping degassed CH2Cl2 solution in a thin quartz tube into a liquid nitrogen Dewar and recording the data after resting it for 3 minutes. The lifetimes for the excited states were measured by a single photon counting spectrometer from Edinburgh Instruments FLS920 with a 360 nm picosecond LED lamp as the excitation source, while those at 77 K were obtained with excitation from a xenon flash lamp. The fluorescent quantum yields (ΦF) were determined in CH2Cl2 solutions at 298 K against quinine sulfate in 1.0 M H2SO4 (ΦFca. 0.55).27 Fast atom bombardment (FAB) mass spectra were recorded on a Finnigan MAT SSQ710 system.

Optical power limiting measurements

The optical power limiting properties of the Pt(II) acetylides were characterized by Z-scan measurements (Fig. S1 in ESI), which were performed at 532 nm for Gaussian mode laser beam with a repetition rate of 20 Hz from a Q-switched Quantel Q-Smart 100 Nd:YAG laser. The laser beam was split into two beams by a beam splitter. One was used as the reference beam, which was received by a power detector (D1), and the other was focused with a lens (f = 25 cm) for the sample measurement. After transmitting through the sample, the light beam entered another power detector (D2). The sample to be measured was moved automatically along a rail to change the incident irradiance on it. The incident and transmitted powers were detected simultaneously by the two power detectors D1 and D2 individually. The OPL performance of each solution sample was measured with ca. 90% transmittance solution at 532 nm in CH2Cl2 filled in a 2 mm quartz cell.

Computational details

Geometrical optimizations were conducted using the popular B3LYP density functional theory (DFT). The basis set used for C, H, B, N and P atoms was 6-311G(d,p), whereas effective core potentials with a LanL2DZ basis set were employed for Pt atom.28,29 The energies of the excited states of the complexes were computed by TD-DFT based on all ground-state geometries. All calculations were carried out by using the Gaussian 09 program.30

Synthesis

The ethynyl aromatic ligands ethynylbenzene, (4-ethynylphenyl)dimesitylborane, 1,3,5-triethynylbenzene, 9-n-butyl-3,6-diethynyl-9H-carbazole and 4,4′,4′′-triethynyltriphenylamine were prepared by the Sonogashira coupling reaction.31–33

General synthetic procedure for Ph-Pt-Cl and B-Pt-Cl

Under N2 atmosphere, a solution of 1,3,5-triethynylbenzene or (4-ethynylphenyl)dimesitylborane (1.0 equiv.) in CH2Cl2 was added slowly to a solution of trans-[PtCl2(PBu3)2] (4.0 equiv.) in CH2Cl2/Et3N (100 mL, v/v = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) in the presence of CuI (0.01 equiv.) catalyst. After addition, the mixture was stirred for another 2 h at room temperature, and then the solvent was removed under reduced pressure and the crude product was purified by silica chromatography with CH2Cl2/petroleum-ether (60–90 °C) (v/v = 1[thin space (1/6-em)]:[thin space (1/6-em)]2) as eluent to give the target product as a solid.
Ph-Pt-Cl: (yield: 58%). 1H-NMR (400 MHz, CDCl3): δ (ppm) 7.27–7.07 (m, 5H), 2.24–1.98 (m, 12H), 1.70–1.56 (m, 12H), 1.52–1.41 (m, 12H), 0.92 (t, J = 7.2 Hz, 18H); 31P-NMR (161.9 MHz, CDCl3): δ (ppm) 8.05 (1JP–Pt = 2373 Hz); FAB-MS (m/z): 735 [M]+. Elemental analysis calcd (%) for C32H59ClP2Pt: C 52.20, H 8.08; found: C 51.96, H 7.93.
B-Pt-Cl: (yield: 59%). 1H-NMR (400 MHz, CDCl3): δ (ppm) 7.37 (d, J = 8 Hz, 2H), 7.19 (d, J = 8 Hz, 2H), 6.81 (s, 4H), 2.30 (s, 6H), 2.04–1.99 (m, 24H), 1.61–1.51 (m, 12H), 1.47–1.39 (m, 12H), 0.91 (t, J = 7.2 Hz, 18H); 31P-NMR (161.9 MHz, CDCl3): δ (ppm) 6.84 (1JP–Pt = 2362 Hz); FAB-MS (m/z): 984 [M]+. Elemental analysis calcd (%) for C50H80BClP2Pt: C 60.00, H 8.19; found: C 59.88, H 8.25.

General synthetic procedure for Pt(II) acetylide complexes

Under N2 atmosphere, each ethynyl-based aromatic ligand was slowly added to a solution of Ph-Pt-Cl or B-Pt-Cl (1.1 equiv. for each ethynyl group) in CH2Cl2/Et3N (v/v = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) with CuI (0.01 equiv.) as the catalyst. The mixture was stirred at room temperature and the reaction was monitored by TLC. After reaction, CuI was removed via a sand-core funnel and the solvent was removed under reduced pressure and the crude product was purified by silica chromatography with CH2Cl2/petroleum-ether (60–90 °C) (v/v = 1[thin space (1/6-em)]:[thin space (1/6-em)]1.5) as eluent to give the product as a white solid in high yield.
TEB-Pt-Ph: (yield: 89%). 1H-NMR (400 MHz, CDCl3): δ (ppm) 7.28–7.26 (m, 6H), 7.19 (t, J = 7.2 Hz, 6H), 7.10 (t, J = 7.2 Hz, 3H), 6.96 (s, 3H), 2.13–2.09 (m, 36H), 1.58–1.54 (m, 36H), 1.46–1.40 (m, 36H), 0.91 (t, J = 7.2 Hz, 54H); 13C-NMR (100 MHz, CDCl3): δ (ppm) 130.80, 130.19, 129.14, 128.02, 127.78, 124.69, 109.02, 108.65, 108.55, 108.40, 108.26, 105.99, 105.84, 105.69, 26.35, 24.45, 24.38, 24.31, 23.99, 23.83, 23.65, 13.86; 31P-NMR (161.9 MHz, CDCl3): δ (ppm) 3.20 (1JP–Pt = 2365 Hz); FAB-MS (m/z): 2248 [M]+. Elemental analysis calcd (%) for C108H180P6Pt3: C 57.66, H 8.06; found: C 57.49, H 8.18.
Cz-Pt-Ph: (yield: 90%). 1H-NMR (400 MHz, CDCl3): δ (ppm) 7.92 (s, 2H), 7.37 (d, J = 8.0 Hz, 2H), 7.29–7.26 (m, 6H), 7.20 (t, J = 7.2 Hz, 4H), 7.10 (t, J = 7.2 Hz, 2H), 4.21 (t, J = 6.4 Hz, 2H), 2.18–2.17 (m, 24H), 1.83–1.78 (m, 2H), 1.65–1.63 (m, 24H), 1.51–1.42 (m, 24H), 1.39–1.35 (m, 2H), 0.93 (t, J = 7.2 Hz, 36H), 0.87–0.83 (m, 3H); 13C-NMR (100 MHz, CDCl3): δ (ppm) 138.63, 130.79, 128.89, 127.80, 124.65, 122.55, 122.30, 119.55, 108.05, 42.86, 31.14, 26.40, 24.50, 24.43, 24.36, 24.12, 23.95, 23.78, 20.53, 13.85; 31P-NMR (161.9 MHz, CDCl3): δ (ppm) 3.03 (1JP–Pt = 2366 Hz); FAB-MS (m/z): 1670 [M]+. Elemental analysis calcd (%) for C84H133NP4Pt2: C 60.38, H 8.02, N 0.84; found: C 60.19, H 8.10, N 0.78.
TPA-Pt-Ph: (yield: 88%). 1H-NMR (400 MHz, CDCl3): δ (ppm) 7.26–7.24 (m, 6H), 7.19 (t, J = 7.2 Hz, 6H), 7.12–7.08 (m, 9H), 6.89 (d, J = 8.4 Hz, 6H), 2.13–2.12 (m, 36H), 1.58–1.57 (m, 36H), 1.48–1.40 (m, 36H), 0.91 (t, J = 7.2 Hz, 54H); 13C-NMR (100 MHz, CDCl3): δ (ppm) 144.61, 131.49, 130.75, 129.13, 127.80, 124.70, 123.55, 123.06, 108.70, 108.40, 108.26, 108.11, 107.04, 106.89, 106.75, 26.33, 24.47, 24.40, 24.33, 23.99, 23.82, 23.65, 13.82; 31P-NMR (161.9 MHz, CDCl3): δ (ppm) 2.81 (1JP–Pt = 2360 Hz); FAB-MS (m/z): 2415 [M]+. Elemental analysis calcd (%) for C120H198NP6Pt3: C 59.63, H 7.88, N 0.58; found: C 59.49, H 7.96, N 0.53.
TEB-Pt-B: (yield: 87%). 1H-NMR (400 MHz, CDCl3): δ (ppm) 7.36 (d, J = 8.0 Hz, 6H), 7.21 (d, J = 8.0 Hz, 6H), 6.95 (s, 3H), 6.81 (s, 12H), 2.30 (s, 18H), 2.11–2.08 (m, 36H), 2.01 (s, 36H), 1.60–1.56 (m, 36H), 1.46–1.37 (m, 36H), 0.89 (t, J = 7.2 Hz, 54H); 13C-NMR (100 MHz, CDCl3): δ (ppm) 141.88, 140.80, 138.21, 136.52, 130.36, 130.21, 128.01, 26.35, 24.44, 24.37, 24.31, 24.00, 23.83, 23.66, 23.40, 21.20, 13.85; 31P-NMR (161.9 MHz, CDCl3): δ (ppm) 3.16 (1JP–Pt = 2351 Hz); FAB-MS (m/z): 2992 [M]+. Elemental analysis calcd (%) for C162H243B3P6Pt3: C 64.98, H 8.18; found: C 64.88, H 8.12.
Cz-Pt-B: (yield: 91%). 1H-NMR (400 MHz, CDCl3): δ (ppm) 7.92 (s, 2H), 7.38 (d, J = 8.0 Hz, 6H), 7.26–7.19 (m, 6H), 6.82 (s, 8H), 4.21 (t, J = 7.2 Hz, 2H), 2.31 (s, 12H), 2.20–2.16 (m, 24H), 2.03 (s, 24H), 1.86–1.78 (m, 2H), 1.65–1.59 (m, 24H), 1.51–1.42 (m, 24H), 1.39–1.35 (m, 2H), 0.93 (t, J = 7.2 Hz, 39H); 13C-NMR (100 MHz, CDCl3): δ (ppm) 141.89, 140.78, 138.63, 138.19, 136.54, 133.17, 130.33, 128.85, 128.00, 122.50, 122.26, 119.43, 114.37, 114.22, 114.08, 110.04, 109.80, 108.08, 103.81, 103.66, 42.85, 31.13, 29.68, 26.36, 24.48, 24.41, 24.35, 24.09, 23.92, 23.75, 23.40, 21.19, 20.52, 13.84; 31P-NMR (161.9 MHz, CDCl3): δ (ppm) 2.98 (1JP–Pt = 2359 Hz); FAB-MS (m/z): 2166 [M]+. Elemental analysis calcd (%) for C120H175B2NP4Pt2: C 66.50, H 8.14, N 0.65; found: C 66.39, H 8.13, N 0.59.
TPA-Pt-B: (yield: 89%). 1H-NMR (400 MHz, CDCl3): δ (ppm) 7.36 (d, J = 8.0 Hz, 6H), 7.21 (d, J = 8.0 Hz, 6H), 7.11 (d, J = 8.4 Hz, 6H), 6.89 (d, J = 8.4 Hz, 6H), 6.81 (s, 12H), 2.30 (s, 18H), 2.14–2.11 (m, 36H), 2.01 (s, 36H), 1.59–1.56 (m, 36H), 1.47–1.40 (m, 36H), 0.90 (t, J = 7.2 Hz, 54H); 13C-NMR (100 MHz, CDCl3): δ (ppm) 144.64, 141.89, 140.79, 138.20, 136.53, 133.08, 131.48, 130.30, 128.01, 123.56, 123.03, 110.10, 26.32, 24.46, 24.39, 24.32, 23.86, 23.69, 23.40, 21.19, 13.80; 31P-NMR (161.9 MHz, CDCl3): δ (ppm) 2.82 (1JP–Pt = 2354 Hz); FAB-MS (m/z): 3159 [M]+. Elemental analysis calcd (%) for C174H252B3NP6Pt3: C 66.11, H 8.03, N 0.44; found: C 66.01, H 7.95, N 0.37.

Results and discussion

Synthesis and structural characterization

In order to obtain the target Pt(II) acetylide complexes, the key mono-substituted complexes Ph-Pt-Cl and B-Pt-Cl should be prepared first. The pathways for the synthesis of Ph-Pt-Cl and B-Pt-Cl are shown in Scheme 1. Through maintaining a large excess in a molar ratio of ca. 4[thin space (1/6-em)]:[thin space (1/6-em)]1 for trans-[PtCl2(PBu3)2] to organic aromatic alkyne, to trans-[PtCl2(PBu3)2], Ph-Pt-Cl and B-Pt-Cl can be easily obtained with a yield of about 60% through Sonogashira cross-coupling. By comparing the number of both aromatic protons and aliphatic ones from the butyl groups in the organic phosphine ligands, the mono-substituted structure of Ph-Pt-Cl and B-Pt-Cl can be clearly indicated. Furthermore, the strong resonance peak at ca. 8.05 and 6.84 ppm with P–Pt coupling effect in the 31P-NMR spectra of Ph-Pt-Cl and B-Pt-Cl indicates the presence of the Pt(PBu3)2 unit. All these spectral data have revealed that Ph-Pt-Cl and B-Pt-Cl are successfully prepared.
image file: c7tc03542j-s1.tif
Scheme 1 Synthesis of Ph-Pt-Cl and B-Pt-Cl.

After obtaining Ph-Pt-Cl and B-Pt-Cl, the target Pt(II) acetylides can be prepared conveniently in a high yield of ca. 90% through Sonogashira cross-coupling between the central aromatic alkyne ligands and Ph-Pt-Cl/B-Pt-Cl (Scheme 2). Compared with 1H-NMR spectra of the central aromatic alkyne ligands, the single resonance peak at ca. 3.2 ppm assigned to the proton in the alkyne groups is absent in the 1H-NMR spectra of these Pt(II) acetylides, indicating the complete coupling between the alkyne groups in the central organic ligands and Ph-Pt-Cl/B-Pt-Cl. In the 1H-NMR spectra of TEB-Pt-Ph and TEB-Pt-B, the single peak at ca. 6.9 ppm is induced by three protons on the central ligand. The single resonance peaks at 6.81 (s, 12H), 2.30 (s, 18H), and 2.01 (s, 36H) ppm can be assigned to –B(Mes)2 groups in TEB-Pt-B. These resonance peaks have also been observed in CAZ-Pt-B and TPA-Pt-B. For CAZ-Pt-Ph and CAZ-Pt-B, NMR signals at ca. 7.9, 7.4 and 4.2 ppm can be ascribed to the carbazole unit. The doublet peak at ca. 6.9 (d, 6H) ppm comes from the central TPA unit of TPA-Pt-Ph and TPA-Pt-B. In the 31P-NMR spectra for all the target Pt(II) acetylides, only one singlet peak at ca. 3.0 ppm with the P–Pt coupling effect can show the presence of the Pt(PBu3)2 unit in the same chemical environment as well as the symmetric configuration of the target Pt(II) acetylides. All these NMR spectral results clearly indicate the successful synthesis of these Pt(II) acetylides.


image file: c7tc03542j-s2.tif
Scheme 2 Synthesis of the target Pt(II) acetylide complexes.

Photophysical properties

The absorption spectra of Pt(II) acetylides have been recorded in CH2Cl2 at 298 K (Fig. 1) and the data are summarized in Table 1. In their UV-vis absorption spectra (Fig. 1), all the Pt(II) acetylides exhibit intense absorption peaks with wavelength peak maximum λmax before 400 nm together with the absorption tail after 400 nm (Fig. 1 and Table 1), indicating their excellent optical transparency in the visible light region (ca. 400–700 nm). Clearly, it seems that the cut-off absorption wavelengths (λcut-off) for the Pt(II) acetylides with the –B(Mes)2 group (ca. 407 nm for TEB-Pt-B, 415 nm for CAZ-Pt-B and 428 nm for TPA-Pt-B) show an obvious bathochromic effect as compared with those for analogous complexes with terminal phenyl group (ca. 358 nm for TEB-Pt-Ph, 375 nm for CAZ-Pt-Ph and 405 nm for TPA-Pt-Ph) (Fig. 1 and Table 1). In addition, the ability of the central aromatic unit for causing the bathochromic effect follows the order of triphenylamine > carbazole > benzene.
image file: c7tc03542j-f1.tif
Fig. 1 UV-vis absorption spectra for the Pt(II) acetylides in CH2Cl2 at 298 K.
Table 1 Photophysical data for Pt(II) acetylides
Compound Absorption λabsa (nm) 298 K Emission λemb (nm) 298 K/77 K Φ F (%) Lifetime of excited statesd S1 state (ns)/T1 state (μs) λ cut-off (nm)
a Measured in CH2Cl2 at a concentration of ca. 10−5 M. sh: shoulder. b The data outside parentheses were obtained in CH2Cl2 at a concentration of ca. 10−5 M at 298 K, while those in parentheses were measured in 5 wt% doped PMMA film with a thickness of ca. 100 nm at 298 K. sh: shoulder. c Measured using quinine sulfate in 1.0 M H2SO4 as the standard. According to UV-vis absorption of the compounds, excitation wavelength was set at 334 nm, while ΦF of the standard is 55%. d The numbers in parentheses are the emission wavelengths of S1 and T1 states. The lifetime of S1 state was measured at 298 K in degassed CH2Cl2 with excitation at 360 nm and that for T1 states was measured at 77 K in the same solvent with the same excitation wavelength.
TEB-Pt-Ph 262, 287, 325sh, 336 366, 462 (399, 466)/465, 500, 516 0.15 0.2 ns (366 nm)/131.8 μs (465 nm) 358
CAZ-Pt-Ph 259, 284, 298, 331, 345 401, 420, 460sh (422, 457)/454, 475, 485, 500 0.15 0.9 ns (420 nm)/44.3 μs (454 nm) 375
TPA-Pt-Ph 262, 282, 321, 374 412, 502 (413, 499)/502, 532, 545, 558 0.20 0.4 ns (412 nm)/116.0 μs (502 nm) 405
TEB-Pt-B 251, 286, 319, 375 416, 511 (419, 506)/503, 532, 561 0.02 0.2 ns (416 nm)/116.8 μs (503 nm) 407
CAZ-Pt-B 249, 300, 328, 373 420, 505 (475sh, 509)/504, 536, 557 0.03 2.7 ns (420 nm)/112.9 μs (504 nm) 415
TPA-Pt-B 252sh, 277, 378 427, 514 (509)/508, 540, 567 0.02 2.0 ns (427 nm)/100.9 μs (508 nm) 428


Compared with TEB-Pt-Ph and TEB-Pt-B, the linear Pt(II) acetylides centered with 1,4-diethynylbenzene ligands DEB-Pt-Ph and DEB-Pt-B show obvious bathochromic effects in their absorption maxima (Fig. S2 in ESI). Representing the structure of one arm in the star-shaped TPA-Pt-B, the reported Pt(II) acetylide B-Pt-N still possesses the maximum absorption wavelength at ca. 372 nm,34 nearly the same to that of TPA-Pt-B (ca. 378 nm). Obviously, the V-shaped CAZ-Pt-Ph and CAZ-Pt-B should be unfavorable for their conjugation extension. Hence, the star- and V-shaped molecular configurations of these Pt(II) acetylides should benefit the transparency of these materials.

In the photoluminescent (PL) spectra of the Pt(II) acetylides in CH2Cl2 solution at 298 K, all complexes exhibit two emission bands (Fig. 2 and Table 1). For CAZ-Pt-Ph, the low-energy emission band appears as an inconspicuous shoulder of the high-energy one (Fig. 2b). The high-energy emission bands come from the singlet excited states (S1) due to their short lifetime (in the order of nanosecond (ns; Table 1)). However, the much longer lifetimes (in the order of microsecond (μs)) together with the large Stokes shift (>100 nm) indicate the triplet excited state (T1) features of the long-wavelength emission bands (Fig. 2 and Table 1). In addition, the substantial intensity enhancement at 77 K also indicates their feature of T1 emission (Fig. 2). Different from the Pt(II) acetylides TEB-Pt-Ph, CAZ-Pt-Ph and TPA-Pt-Ph, the ones with –B(Mes)2 group exhibit a blue-shifted triplet emission at 77 K compared with that at 298 K in each case (Fig. 2). Comparing the PL spectra of these Pt(II) acetylides at 298 K (Fig. 2), it can be noted clearly that introducing the –B(Mes)2 group can effectively enhance emission from T1 states, since the Pt(II) acetylides TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B exhibit PL spectra dominated by triplet emission (Fig. 2). This trend is also supported by their much lower quantum yield at S1 states (ΦF) than that of TEB-Pt-Ph, CAZ-Pt-Ph and TPA-Pt-Ph (Table 1). Hence, introduction of the –B(Mes)2 group can effectively increase the quantum yield of T1 states, which benefits OPL performance of these Pt(II) acetylides for a nanosecond laser based on reverse saturable absorption (RSA) of T1 states.


image file: c7tc03542j-f2.tif
Fig. 2 Photoluminescent (PL) spectra for the Pt(II) acetylides in CH2Cl2 solution at both 298 K and 77 K.

Besides, in solution, PL spectra of these Pt(II) acetylides in 5 wt% doped PMMA film with a thickness of ca. 100 nm have been obtained (Table 1 and Fig. S3, ESI). Importantly, the triplet emission bands of these Pt(II) acetylides are greatly enhanced in the solid film compared with those in solution (Fig. S3 and Fig. 2, ESI). This result might be ascribed to the following facts: (1) the rigid solid PMMA matrix can effectively restrain the twisting of the molecular skeleton of these acetylides to facilitate intersystem crossing (ISC) process from singlet states (S1) to triplet states (T1) and hence enhance T1 emission. (2) In addition, restraining the twisting of the molecular skeleton of these acetylides can also promote radiative decay of T1 states to enhance the T1 emission.

In order to interpret the photophysical properties of the aforementioned Pt(II) acetylides, optimized gas-phase geometry essentially depends on the initial configuration used in the calculations and their frontier molecular orbitals and transition characters showing a close relationship with their photophysical properties are obtained by time-dependent density functional theory (TD-DFT) calculations. Owing to both the large contribution to the transition of S1 states and high configuration interaction (CI) coefficient of ca. 0.7 (Table 2), the transitions between the key molecular orbitals can represent the feature of S1 states of the Pt(II) acetylides. From the molecular orbital patterns corresponding to HOMO → LUMO (H → L) or HOMO → LUMO+1 (H → L+1) transitions (Fig. 3), S1 states of TEB-Pt-Ph, CAZ-Pt-Ph and TPA-Pt-Ph mainly exhibit ligand-centered π–π* characteristics. Differently, S1 states of TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B display intra-ligand charge transfer (ILCT) features from the π orbitals of the central organic ligands to the π* orbitals of the –B(Mes)2 group, especially the pπ* orbital on the boron atom (Fig. 3). Clearly, the electron-accepting empty pπ orbital on the boron atom can facilitate electron transition processes in TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B to give S1 states with a lower energy level. Hence, as indicated in Fig. 1, the major absorption bands of TEB-Pt-B (ca. 375 nm), CAZ-Pt-B (ca. 373 nm) and TPA-Pt-B (ca. 378 nm) can exhibit an obvious bathochromic effect as compared with those of their analogues with the terminal phenyl group (ca. 336 nm for TEB-Pt-Ph, 345 nm for CAZ-Pt-Ph and 374 nm for TPA-Pt-Ph) (Fig. 2 and Table 1). The theoretical and experimental results can coincide well with each other, indicating the validity of the theoretical computations.

Table 2 Contribution of the metal dπ orbitals to HOMO and LUMO together with the TD-DFT calculation results
Compound Contribution of dπ orbitals to HOMOa Contribution of dπ orbitals to LUMO/LUMO+1a Largest coefficient in the CI expansion of the T1 stateb Largest coefficient in the CI expansion of the S1 stateb Percentage contribution of the transition to the S1 stateb Oscillator strength (f) of the S1 ← S0 transition
a The data have been obtained by exporting DFT results with the software AOMix. b H → L represents HOMO to LUMO transition. CI stands for configuration interaction.
TEB-Pt-Ph 23.2% 16.7%/13.5% H → L (0.36777)

439 nm

H → L+1 (0.68936)

312 nm

95.0% 0.0823
CAZ-Pt-Ph 14.7% 0.9%/14.8% H → L+1 (0.41709)

433 nm

H → L (0.67947)

346 nm

95.2% 0.0214
TPA-Pt-Ph 8.9% 9.5%/7.4% H → L (0.6056)

478 nm

H → L (0.69599)

386 nm

94.3% 1.0671
TEB-Pt-B 22.7% 0.9%/0.7% H → L+1 (0.37486)

483 nm

H → L+1 (0.65977)

406 nm

95.6% 0.0033
CAZ-Pt-B 11.0% 1.7%/1.1% H → L+1 (0.40602)

501 nm

H → L (0.69241)

433 nm

95.3% 0.2847
TPA-Pt-B 9.0% 3.9%/3.5% H → L (0.50850)

533 nm

H → L (0.69402)

491 nm

94.9% 0.3021



image file: c7tc03542j-f3.tif
Fig. 3 Key molecular orbitals involved in S1 and T1 transitions of the Pt(II) acetylides. (a) TEB-Pt-Ph, (b) CAZ-Pt-Ph, (c) TPA-Pt-Ph, (d) TEB-Pt-B, (e) CAZ-Pt-B and (f) TPA-Pt-B.

Based on the heavy atom effect, T1 emission can typically be detected due to the spin–orbit coupling (SOC) effect, which can promote the formation of T1 states through intersystem crossing (ISC) process. It has been shown that the effective mixing among d orbitals of the metal centers and the π orbitals of the organic ligands can enhance the SOC effect in transition metal acetylides.36 In this way, the transition processes in transition metal acetylides show more obvious metal-based characters rather than just the ligand-centered π–π* features. From the MO patterns of these Pt(II) acetylides (Fig. 3), the dπ orbitals of the Pt(II) centers are noticeably involved in HOMOs of these Pt(II) acetylides, while LUMOs are mainly located on the π* orbitals of the organic ligands. In addition, dπ orbitals of the Pt(II) centers can contribute to LUMO+1 of the Pt(II) acetylides (Table 2). All these results indicate the effective mixing between orbitals of both Pt(II) centers and the corresponding organic ligands to enhance the SOC effect. Based on the theoretical data in Table 2, H → L or H → L+1 transitions can represent the features of T1 transitions to a large extent. Hence, strong SOC effect should appear in these Pt(II) acetylides. Accordingly, all of them show T1 emission bands even at 298 K (Fig. 2). Owing to its strong electron-accepting ability, π* and pπ* orbitals of the –B(Mes)2 group can give a major contribution to LUMOs of TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B. On the contrary, the terminal phenyl group in TEB-Pt-Ph, CAZ-Pt-Ph and TPA-Pt-Ph cannot contribute to their LUMOs. So, the –B(Mes)2 group can enhance the orbital mixing between the Pt(II) center and the organic ligand, especially the –B(Mes)2 group. Hence, the SOC effect is stronger in TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B. This conclusion is supported by the much stronger T1 emission bands of TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B than those of their counterparts, TEB-Pt-Ph, CAZ-Pt-Ph and TPA-Pt-Ph, respectively (Fig. 2). From the MO patterns of TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B terminated with –B(Mes)2 group (Fig. 3), their T1 states show obvious charge-transfer features. Hence, their T1 emission shows blue-shift effect at 77 K (Fig. 2).37 On the contrary, this trend cannot be observed in TEB-Pt-Ph, CAZ-Pt-Ph and TPA-Pt-Ph (Fig. 2).

Optical power limiting behavior

Owing to the strong SOC effect induced by heavy metal centers, the emission signals from the decay of the T1 states can even be observed at 298 K in these Pt(II) acetylides, especially for TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B (Fig. 2). The effective formation of the T1 states provides these Pt(II) acetylides with OPL behavior, as induced by the strong absorption ability of their T1 states, especially for a nanosecond laser pulse. In addition, Pt(II) acetylides with star configuration have been rarely employed to investigate their OPL properties. So, it is desirable to study the OPL properties of these Pt(II) acetylides with the view of evaluating their potential as new OPL materials and providing structure–property information for guiding the design of efficient Pt(II) acetylides for OPL. Bearing this in mind, the OPL properties of our new Pt(II) acetylides have been investigated.

All the Pt(II) acetylides exhibit exceptional optical transparency at 532 nm, indicating their extremely low ground-state absorption at this wavelength (Fig. 1). This absorption behavior is a primary requirement for characterizing their OPL properties against 532 nm laser beam. The OPL behaviors of these Pt(II) acetylides have been characterized in CH2Cl2 with a linear transmittance To of ca. 90% (Fig. 4) through the open-aperture Z-scan method. The incident irradiance upon the Pt(II) acetylide solution sample is changed by its distance (Z-position) from the focal point, where Z = 0 along the Z direction (Fig. S1 in ESI). The closer to the focal point, the higher incident irradiance on the sample can be achieved and the incident irradiance reaches the highest value at the focal point (Z/Zo = 0).


image file: c7tc03542j-f4.tif
Fig. 4 Open-aperture Z-scan results for the Pt(II) acetylides (To at ca. 90%).

From the Z-scan curves (Fig. 4), it can be clearly seen that transmittance (T) of the Pt(II) acetylide solution remains almost constant when it is far from the focal point and the incident irradiance upon the sample is weak, showing a linear optical property (i.e. obeying Beer's law). It indicates that the Pt(II) acetylides exhibit linear optical behavior under weak incident laser irradiance. However, when the samples are moved towards the focal point to increase incident irradiance upon them, T of the samples decreases to show a nonlinear optical effect, i.e. OPL behavior (Fig. 4). In both series of Pt(II) acetylides, the OPL ability follows the order of TPA-Pt-Ph > CAZ-Pt-Ph > TEB-Pt-Ph and TPA-Pt-B > CAZ-Pt-B > TEB-Pt-B based on the central ligands. Obviously, triphenylamine and carbazole units exhibit higher electron-donating feature than benzene. It appears that electron-donating ligands benefit the OPL properties of Pt(II) acetylides, which has been observed in their linear analogues.34 For the Pt(II) acetylides with the same central ligands, the ones with the –B(Mes)2 terminal group show stronger OPL response than their counterparts terminated with the phenyl group. Owing to the good fitting of the Z-scan data with RSA mechanism (Fig. S4 in ESI), the OPL behaviors of these Pt(II) acetylides are induced by RSA of their excited states.22,25 Based on this assumption, this result can be explained as follows: The molecules of the Pt(II) acetylides in the ground state (S0) are excited by laser irradiance to the first singlet state (S1), which can go to the first triplet state (T1) through intersystem crossing (ISC) process owing to the SOC effect induced by the Pt(II) centers (Fig. 5). The ISC process is so fast that the molecules in the long-lived T1 state can reach a certain population easily and then absorb laser energy strongly to reach the higher triplet state (Tn) in a laser pulse duration to induce OPL effect (Fig. 5). The lifetime of S1 state is very short (<1 ns) (Table 1). Hence, the optical absorption accompanied with S1 → Sn transition can give a negligible contribution to nanosecond OPL response in these Pt(II) acetylides. Clearly, if the proposed triplet RSA mechanism is valid, the complexes with higher T1 emission ability should show a better OPL performance.4,25,34,35 From Fig. 2, the Pt(II) acetylides TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B exhibit much stronger triplet emission bands than their counterparts at 298 K. Hence, they show enhanced OPL response relative to their counterparts TEB-Pt-Ph, CAZ-Pt-Ph and TPA-Pt-Ph, respectively (Fig. 4). It can also be noted that TPA-Pt-Ph and TPA-Pt-B with the central triphenylamine ligand show much stronger OPL effect than the other Pt(II) acetylides. From the fine structures in the low-temperature PL spectra (Fig. 2), it can be concluded that T1 states of these Pt(II) acetylides exhibit mainly ligand-centered π–π* characteristics, which is also indicated by the MO patterns (Fig. 3). It has been shown that the ligand-centered T1 states from different organic ligands can show different OPL ability.26 Hence, the much higher OPL activity of TPA-Pt-Ph and TPA-Pt-B can be ascribed to the high absorption ability associated with triphenylamine-based T1 states.


image file: c7tc03542j-f5.tif
Fig. 5 Optical power limiting mechanism involved in the Pt(II) acetylides.

In order to evaluate the potential of these Pt(II) acetylides as OPL materials, the OPL performance of TPA-Pt-B is compared with that of C60, a state-of-the-art RSA OPL material. At a high linear transmittance, To, of ca. 90%, TPA-Pt-B shows better OPL ability than C60 and TPA-Pt-Ph exhibits a slightly higher OPL ability than C60 (Fig. 6), showing their great potential in the field of OPL. Besides their good OPL ability, all the Pt(II) acetylides exhibit maximum absorption wavelength before 400 nm (Fig. 1 and Table 1), indicating their exceptional optical transparency in the visible light region (ca. 400–700 nm). However, C60 possessing broad and weak absorption in the visible light region shows poor transparency with purple color in toluene. Typically, for traditional Pt(II) acetylides with diethynyl ligand, the central ligand can only possess two Pt(II) centers.26 However, the star-shaped Pt(II) acetylides in this work can bear three Pt(II) centers. Based on the SOC theory, the high density of Pt(II) center promotes SOC effect to benefit OPL ability of the concerned Pt(II) acetylides. Furthermore, in linear Pt(II) acetylides, increasing the number of Pt(II) centers generally extends molecular conjugation and induces red-shift of the maximum absorption wavelength to show poor transparency,25 despite the fact that conjugation expansion shows a positive effect on the OPL performance. Besides, the obviously improved optical transparency mentioned above as compared with their linear counterparts (Fig. S2 in ESI), TEB-Pt-Ph and TEB-Pt-B can still show comparable OPL ability to that of DEB-Pt-Ph and DEB-Pt-B with linear molecular configuration, respectively (Fig. S5 in ESI). The advantage of reverse saturable OPL materials can be characterized to some extent by the figure of merit factor σex/σo = ln[thin space (1/6-em)]Tsat/ln[thin space (1/6-em)]To, where σex is the effective excited-state absorption cross-section, σo is the ground-state absorption cross-section, Tsat is the transmittance at the saturation fluence and To is the linear transmittance. From the Z-scan results, the σex/σo values for TPA-Pt-Ph, TEB-Pt-B, CAZ-Pt-B and TPA-Pt-B are 10.8, 8.5, 9.6 and 11.6, respectively. They are all higher than that of the state-of-the-art RSA OPL absorber C60 (ca. 5.9). In addition, even for TEB-Pt-Ph and CAZ-Pt-Ph, the σex/σo values (ca. 5.2 for TEB-Pt-Ph and 5.0 for CAZ-Pt-Ph) are comparable to that for C60 (ca. 5.9). The high σex/σo values associated with these Pt(II) acetylides can be ascribed to their high optical transparency at 532 nm. Taking their good OPL behavior and exceptional transparency into consideration, the star-shaped Pt(II) acetylides with the proper central ligand provide a feasible strategy for fulfilling the optimized trade-off between OPL activity and optical transparency for OPL materials based on RSA mechanism.


image file: c7tc03542j-f6.tif
Fig. 6 Comparison of OPL properties of TPA-Pt-Ph, TPA-Pt-B and C60 at the same linear transmittance (To at ca. 90%).

Conclusions

Several star- and V-shaped Pt(II) acetylides have been successfully prepared with –B(Mes)2 and phenyl units as the terminal groups. Compared with the terminal phenyl group, the electron-accepting –B(Mes)2 group exhibits much higher capacity for enhancing triplet emission in Pt(II) acetylides through a more effective mixing of MOs of the organic ligands and the Pt(II) centers to benefit their OPL performance. Hence, Pt(II) acetylides show decent OPL effect and OPL performance of certain Pt(II) acetylides is even better than that of the state-of-the-art OPL material C60, indicating their potential in OPL application. Importantly, the molecular configurations employed by these Pt(II) acetylides show the merit of increasing the number of Pt(II) centers while maintaining optical transparency of the concerned Pt(II) acetylides. So, this research should offer a feasible strategy towards OPL/transparency trade-off optimization for OPL materials based on RSA mechanism.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was financially supported by the Fundamental Research Funds for the Central Universities (cxtd2015003), the Key Creative Scientific Research Team in Yulin City of Shaanxi Province (2015cxy-25), the China Postdoctoral Science Foundation (Grant no. 20130201110034, 2014M562403), and the National Natural Science Foundation of China (Grant no. 20902072, 21572176). The financial supporting from State Key Laboratory for Mechanical Behavior of Materials is also acknowledged. W.-Y. W. acknowledges the financial support from Hong Kong Research Grants Council (PolyU 12338416), National Natural Science Foundation of China (Project no. 51573151) and the Hong Kong Polytechnic University (1-ZE1C).

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Footnote

Electronic supplementary information (ESI) available: Sketches of Z-scan measurement, Z-scan data and theoretical fitting results, and absorption spectra and chemical structures for DEB-Pt-Ph and DEB-Pt-B. See DOI: 10.1039/c7tc03542j

This journal is © The Royal Society of Chemistry 2017