J. MATER. CHEM., 1991, 1(6), 1041-1043 1041 Pressure-sensitive Absorption Spectra of Thin Films of Bis(diphenylglyoximato)platinum(ii), Pt(dpg),: Potential Application as an Indicator of Pressure lchimin Shirotani," Yukio Inagaki," Wataru Utsumib and Takehiko Yagib "Muroran Institute of Technology, 27-1, Mizumoto, Muroran-shi 050, Japan bThe Institute for Solid State Physics, The University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan The optical properties of thin films of Pt(dpg), have been studied at high pressures. Two absorption bands of the complex in the visible region shifted sharply to longer wavelengths with increasing pressure. The rates of the red shift with pressure were ca. -1900 cm-' GPa-' for the metal-to-ligand charge-transfer(M-L) band and -3000 cm-' GPa-' for the 5d-6p band.These values are the largest in the known one-dimensional dB metal complexes. The colours of the thin films in the diamond-anvil pressure cell were observed visually at various pressures. The colours of Pt(dpg), turned from red-brown at atmospheric pressure to brown at 0.27 GPa, green at 0.69 GPa, to yellow-green at 1.24 GPa and yellow at 1.92 GPa. This material can be utilized as an indicator of pressure over a GPa range 0-2. Keywords: Optical activity ; High pressure ; Thin film The d8 metal complexes with various kinds of 1,2-dionediox- imes crystallize in a columnar structure. The molecules of these complexes stack one above the other with rotation of 90" which forms a linear chain. Bis( 1,2-dionedioximato)M1' (M =Ni, Pd, Pt) complexes are chemically stable, and have various fine colours at atmospheric pressure.Absorption bands of these complexes show a remarkable shift to longer wavelengths with increasing In addition, a new pressure-induced absorption band is observed for the Pt complexe~.~.~The colours of bis(1,2-dionedioximato)M1' change with the pressure shift of the absorption bands. For example, the colours of the nioxime ligand bis( 1,2-cyclohexane- dionedioximato)Pd", Pd(niox),, turn from yellow through orange, red, purple, blue, to green and pale yellow with increasing pressure.8 If the relationship between colour and pressure is studied in detail, a semiquantitative value of pressure could be obtained from the visual observation of the change in colour with pres~ure.~*~ This can be utilized as an indicator of pressure by a colorimetric method similar to pH testing paper. We have already found a pressure-sensitive platinum com- plex, bis(diphenylglyoximato)platinum(II), Pt(dp&.' A mol-ecular structure of the complex is shown in Fig.l. Evaporated films of bis( 1,2-dionedioximato)M1' complexes are easily pre- pared in UUCUO.~~The optical properties of thin films of Pt(dpg), have been studied at high pressures. This material can be used as an indicator of pressure in the low-pressure region. Fig. 1 Molecular structure of bis(dipheny1glyoximato)M" (M =Ni, Pd, Pt), M(dpgh Experimental The development of diamond-anvil pressure cells enables the effect of the pressure on various materials to be observed in situ.Using this cell, photographs of a thin film of Pt(dpg), were taken at various pressures. The absorption spectra of the complex were measured simultaneously at high pressures. An optical system consisted of a standard microscope and a monochromator with an associated photodetection system. This apparatus is shown in Fig. 2. Pressure was determined from pressure shift in the sharp R-line fluorescence spectrum 1 x-y recorder (6) 35mm camera (9) photon counter (8) photom -17 no 0 iI 0 I (7)spectrometer (3) objectives (5) He-Cd laser diamond cell (4) condenser (1) microscope Fig. 2 Optical system with high pressure apparatus J. MATER.CHEM., 1991, Vol. 1 film 3 -I single crystal .1 !I needle axis I I I I 25 20 15 10 v/103 crn-' Fig. 3 Polarized reflectance spectra of a single crystal of Pt(d~g)~ of ruby." Water was used as the pressure-transmitting medium. Pt(d~g)~was prepared by stirring a mixture of an aqueous solution of K2PtC1, and a hot alcoholic solution of diphenylglyoxime (dpg). The complex was purified by recrys- tallization. The thin films of the complex were prepared by evaporation onto substrates in a vacuum of ca. Torr.t" The thickness of the films was monitored by means of a quart z-crys t a1 oscillator. Results and Discussion The platinum ions in the square-planar d8 complex shown in Fig. 1 are surrounded by four nitrogen atoms of two diphenylglyoxime anions. The d orbitals of the platinum ion are split by a crystal field of symmetry. The eight electrons of the platinum ion core fill the dz2, d,,, d,, and d,, states.Crystalline spectra of bis(dimethylglyoximato)M", M(dmg),, have been studied at atmospheric pressure. l2 Two character- istic bands in the crystalline state are observed in the visible region. The band polarized parallel to a column is ascribed to the nd -(n + 1)p (n=3, 4, 5) transition. On the other hand, the band polarized perpendicular to a column is assigned to the metal-to-ligand charge-transfer (M-L) transition. Fig. 3 shows the polarized reflectance spectra of a single crystal of Pt(dpg),. The 394 nm (25 400 cm-') band was polarized per- pendicular to a needle axis.Thus, the band is ascribed to the M-L transition. The 550 nm (1 8 200 cm-') band was polar- ized parallel to the needle axis. This band is due to the 5d-6p transition. The 394 and 550 nm bands in the film correspond to both bands of the single crystal. Fig.4 shows absorption spectra of the thin film and the CHC13 solution of Pt(dpg)z. The 290 nm band in the film corresponds to 280 nm band in the solution. This band is ascribed to a n-n* transition in the ligand. The d-p band observed in the crystalline spectra was not observed in solution. Fig. 5 shows the absorption spectra of Pt(dpg), at high pressures. The sample was evaporated directly onto the surface of the diamond-anvil in high vacuum. The film thickness was ca.3000A. Absorption spectra of the film were measured t 1 Torr = 133.322 Pa I 2 00 600 800 400 i/nm (-)Fig.4 Absorption spectra of Pt(d~g)~, thin film; (---) CHCI, solution 400 500 600 700 800 A/nm Fig. 5 Absorption spectra of Pr(dpg), thin film at (-) 0, (---) 0.27, (.....) 0.69, (-.-) 1.24, (--) 1.92 GPa over the 0-2 GPa region. The absorption peaks of the M-L and d-p bands shifted sharply to longer wavelengths with increasing pressure. The rates of the red shifts with pressure were -1900cm-' GPa-' for the M-L band and -3000 cm-' GPa-' for the d-p band. These values are the largest in the known one-dimensional d8 metal complexes. High-pressure absorption spectra of the KBr disc of Pt(dpg), have been studied previou~ly.~ The pressure shifts for the thin film are much greater than those for the KBr disc.This tendency is also observed for Pd(ni~x)~. The colours of the film changed markedly with the red shift of the bands at high pressures. We have observed visually the sample in the dia- mond-anvil cell at high pressures. Plate l shows a series of photographs of Pt(dpg), at various pressures. The colours of the complex changed from red-brown at atmospheric pressure through brown at 0.27 GPa, green at 0.69 GPa, yellow-green at 1.24 GPa to yellow at 1.92 GPa. If the relationship between colour and pressure is studied in detail, a semiquantitative value of pressure can be obtained from the visual observation of the change in colour with pressure. This can be utilized as an indicator of pressure.It should be noted that the colour tone depends on a number of factors including light source, sample thickness, pressure-transmitting medium and pressure distribution. Pd(niox), turns from yellow to orange and then to success- ive colours with increasing pressure.8 This complex has an absorption band based on the d-p transition in the visible region. The d-p band shifts from 477 nm at atmospheric pressure to 700 nm at 6 GPa. The rate of the red shift is ca. -1100 cm- GPa-', much smaller than that of Pt(dpg)z. Pd(niox), becomes an excellent pressure indicator over the range 0-8GPa. On the other hand, Pt(d~g)~ has M-L and d-p bands in the visible region. These bands showed very J.MATER. CHEM., 1991, VOL. 1 Plate 1 Pt (dpg), at (a)atmospheric pressure; (b)0.27 GPa; (c) 0.69 GPa; (d) 1.24 GPa; (e) 1.92 GPa (Facing p. 1042)I. Shirotani et al. J. MATER. CHEM., 1991, Vol. 1 large red shifts with pressure. Thus, Pt(dpg), can be utilized as a pressure indicator in the low-pressure region. The crystal structure of Pt(dpg), has not been studied yet at high pressures. However, we have reported previously the results of the crystal structure of Pt(dmg), at high pressures.2 The lattice constants shrink ca. 9% for the c axis (needle axis) and ca. 4.5% for the a and b axes, up to 4GPa. Similar results are expected for Pt(dpg),. The direction of transition for the d-p band is parallel to the needle axis with Pt-Pt bonds. The dZ2 and p orbitals extend to the direction of the needle axis.Thus, the d-p band is very sensitive to pressure. On the other hand, the direction of the transition for the M- L band is perpendicular to the needle axis. The M-L band is the transition from d,, and d,, orbitals in the central metal to the 7c* orbital in the ligand. Since the needle axis of Pt(dpg), is more easily compressed than the other axes, the pressure shift of the d-p band is much greater than that of the M-L band. The pressure shifts of the d-p bands of M(niox),, M(dmg), and M(dpg), are given in Table 1. These are approximate mean values over the range 0-2 Gpa and are very large. The rate of red shift with pressure increases in the order, Ni complexes <Pd complexes, <Pt complexes and in the order (niox) complexes <(dmg) complexes <(dpg) complexes.Thus, the peak shift of Pt(dpg), is the largest in known (1,2-Table 1 Rates of red shift (cm-' GPa-') of the d-p bands of bis(l,2-dionedioximato)M(n) complexes at high pressure M(niox), Ni -820 -840 -1300 Pd -1 130 -1130 -1940 Pt -2400 -2600 -3000 dionedioximato)M" complexes. The crystal packing of M(dpg), is very loose because of the bulky phenyl group. The M-M distances of M(dpg), are greater than those of M(dmg), and M(niox),.13 Thus, the physical properties of M(dpg), are very sensitive to pre~sure.'~ The authors thank Professor M. Tanaka, Nagoya University, for the measurement of polarized reflectance spectra of the single crystal of Pt(dpg),.References 1 L. E. Godycki and R. E. Rundle, Acta Crystallogr., 1953, 6, 487. 2 M. Konno, T. Okamoto and I. Shirotani, Acta Crystallogr., Sect. B., 1988, 45, 142. 3 J. C. Zahner and H. G. Drickamer, J. Chem. Phys., 1960, 33, 1625. 4 M. Tkacz and H. G. Drickamer, J. Chem. Phys., 1986, 85, 1184. 5 Y. Hara, I. Shirotani and A. Onodera, Solid State Commun., 1976, 19, 181. 6 I. Shirotani and T. Suzuki, Solid State Commun., 1986, 59, 533; I. Shirotani, M. Konno and Y. Taniguchi, Synth. Met., 1989, 29, F123. 7 I. Shirotani, A. Kawamura, K. Suzuki, W. Utsumi and T. Yagi, Bull. Chem. SOC. Jpn., 1991, 64, 1607. 8 I. Shirotani, Platinum Met. Rev., 1987, 31, 20. 9 I. Shirotani, Gendai Kagaku, 1987, 191, No 2, 30. 10 I. Shirotani, N. Minobe, Y. Ohotsuki, H. Yamochi and G. Saito, Chem. Phys. Lett., 1988, 147, 231. 11 J. D. Barnett, S. Block and G.J. Piermarini, Rev. Sci. instrum., 1973,44, 1. 12 Y. Ohashi, I. Hanazaki and S. Nagakura, inorg. Chem., 1970, 9, 2551. 13 A. S. Foust and R. H. Soderberg, J. Am. Chem. SOC., 1967, 89, 5507. 14 I. Shirotani, K. Suzuki, T. Suzuki, T. Yagi and M. Tanaka, submitted. Paper 1/02752B; Received 10th June, 1991