J. Phys. Chem. 1985,89, 5649-5654 order to probe the NO(A,v’=OcX,v”= 1) transition. The resulting spectra exhibit the same highly excited rotational distribution of J levels as observed previously.’ Some experiments have also been performed with the REMPI technique to show the validity of the method. It should be pointed out that, in both cases, care must be taken in the choice of the wavelengths, to avoid processes which would alter the spectra of the studied transition and thus the rotational distribution of the nascent products. Conclusion
W e have shown that the two-photon excitation of the NO (A2Z+) state can be achieved using two different colors. The spectroscopic study is performed by monitoring the total fluorescence of the N O A state (TPF) or the ion current of the
5649
ionized NO(A2Z+) state via a one- or two-photon ionization process. When the one-photon ionization process of the AZZ+ vibrational level fulfills the Au = 0 selection rule, the REMPI and TPF spectra are identical and resonances which perturb the one-color spectra are avoided. The present method thus provides an efficient way to obtain spectra in which rotational transition energies and intensities are correctly reproduced by theoretical calculations. When no direct ionization is expected, irregularities such as enhanced or vanishing rotational lines can always occur. The two-color, two-photon excitation of NO has been shown to offer a very convenient method to probe the rovibronic energy distribution when NO is produced in a photofragmentation reaction. Registry No. NO, 10102-43-9;methyl nitrite, 624-91-9
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The Paramagnetic Metal Effect on the Ligand Localized S1 T1 Intersystem Crossing in the Rare-Earth-Metal Complexes with Methyl Salicylate Seiji Tobita,* Gunma Technical College, Toribacho, Maebashi, Gunma, Japan
Masayuki Arakawa, and Ikuzo Tanaka Department of Chemistry, Tokyo Institute of Technology, Ohokayama. Meguro, Tokyo, Japan (Received: August 9, 19%5)
The electronic relaxation processes in the chelates of La3+,Gd3+,Tb3+,and Lu3+with methyl salicylate have been investigated by measurements of picosecond fluorescence, nanosecond transient absorptions, and quantum yields. In the Tb3+complex, only the characteristic emission bands from the central metal ion are observed as a result of rapid intramolecular energy transfer. The other three metal complexes, which have no available energy levels for intramolecular energy transfer, show a rather different behavior which depends on the properties of the central metal ions. The quantum yields of the S, TI intersystem crossing are not appreciably altered by a change of the central metal ions. However, the fluorescence lifetimes are decreased dramatically in the paramagnetic Gd3+ (240 ps) and Tb3+ (e10 ps) complexes compared with those in the diamagnetic La3+(2.2 ns) and Lu3+ (2.4 ns) complexes. The rate constants derived from these results for the SI TI intersystem crossing, kTM,in ligands are 5.5 X lo7, 7.5 X lo8, and 7.9 X lo7 s-I for the La”, Gd3+,and Lu3+complexes, is observed in the paramagnetic Gd3+complexes, which can be attributed to the electron respectively. A large increase of ~ T M exchange mechanism with ligand 7r electrons.
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Introduction Recently, inorganic photochemistry has been developed as one of the most important subjects associated with solar energy conversion and biochemical substances. Particular attention has been paid to the excited states of transition-metal and rareearth-metal complexes. The rare-earth metals are characterized by 4f electrons which are shielded from their environment by an outer core of 5s and 5p electrons. So in general the atomic properties of the rare-earth metal ions are almost always retained after formation of complexes through organic ligands, which is in marked contrast with the transition-metal complexes. Thus electronic excitation of rare-earth complexes to the lowest excited singlet state (SI)of a ligand results in the observation of complex luminescence, consisting of molecular fluorescence, molecular phosphorescence, and atomic line emissions of lanthanide ions. The ratios of these emission intensities are strongly dependent on the kind of rare-earth element involved. Weissman found that photoexcitation of an electron associated primarily with a ligand in an europium chelate results in the observation of atomic line emission of an Eu3+ ion with a high yield.’ His interpretation was that direct excitation of the metal ion was not responsible for the line emission but rather intra(1) Weissman, S. I. J . Chem. Phys. 1942, 10, 214.
0022-3654/85/2089-5649$01.50/0
molecular energy transfer took place from an excited state of the ligand to the localized intra-4f shell energy levels of the metal ions. Subsequently, the spectroscopic properties of rare-earth complexes have been studied by several researchers to elucidate the mechanism of the intramolecular energy transfer in solution or in the crystalline state.” Various properties of the rare-earth complexes have been revealed as a r e s ~ l t . ~ Schematic energy diagrams for the rare-earth complexes are illustrated in Figure 1. The ff* levels of the central metal ion are located at energetically lower states (case a) or higher (case c) than the lowest triplet state (TI) of a ligand.6 No ff* levels exist in the case of La3+and Lu3+ (case b), because the 4f orbitals are vacant in the former and fully occupied in the latter. From the studies of the intramolecular energy transfers quoted above, it has been found that the x electrons in ligands strongly interact (2) Brown, A.; Wilkinson, F. J . Chem. SOC., Faraday Trans. 2 1979, 75,
880. (3) Hayes, A. V.; Drickamer, H. G. J . Chem. Phys. 1982, 76, 114. (4) Richter-Lustig, H.; Ron, A.; Speiser, S. Chem. Phys. Lett. 1982, 29, 516. ( 5 ) Reisfeld, R.; Jargensen, K. “Lasers and Excited States of Rare Earths”; Springer-Verlag: West Berlin, 1977; Chapter 4. (6) Stanley, E.C.; Kimeberg, B. I.; Varga, L. P. Anal. Chem. 1966, 38, 1362.
0 1985 American Chemical Society
Tobita et al.
5650 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
complexes of a series of transition and/or rare-earth metals. Recently, the authors have verified the “paramagnetic effect” for the T I Soradiative process by the quantitative determination of the radiative rate constants in rare-earth complexes with benzoyltrifluoroacetone.I6 This has been interpreted theoretically as stronger singlet-triplet mixing resulting from an exchange interaction between the ligand and metal electrons. In this study, kinetic analyses of the excited states (SIand TI) have been performed to elucidate the relaxation mechanisms and also to test the “paramagnetic effect” for the SI w-+ TI intersystem crossing in the rare-earth (La, Gd, Tb, Lu) complexes with methyl salicylate (MS). The fluorescence lifetimes were measured by a picosecond pulsed laser excitation followed by the time-resolved detection of the emission. A nanosecond laser flash photolysis system was used to determine the triplet-state lifetime. As a result, the paramagnetic character of the central metal ion is found to enhance not only the T, So radiative rate but also the SI --+ TI intersystem crossing rate from the localized electronic states in ligands.
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+
-11‘
Experimental Section The methyl salicylate complexes Ln(MS)3 and La3+, Gd3+, Tb3+,and Lu3+ were all prepared in a similar way by standard techniques using P-diketonato complexes.” Elemental analyses were performed to check the purities of the samples and the results were in good agreement with the expected formulae. The comM in mixed pounds were dissolved at a concentration of solvent EME (8 parts ethanol, 2 parts methanol, and 1 part diethyl ether by volume). Methanol (products from Wako) and diethyl ether (products from Merck) were of spectroscopic grade and used without further purification. Ethanol (products from Wako) was fractionally distilled over sodium to remove traces of water. All samples were thoroughly degassed by repeating freeze-pumpthaw cycles in a high vacuum line before use. Absorption and emission spectra were taken with a Jasco UVIDEC 5 10 spectrophotometer and a Jasco FP-550A spectrofluorometer, respectively. The spectral response of the spectrofluorometer was obtained by use of standard fluorescence solut i o n ~ . ’ ~The , ~ fluorescence ~ quantum yields were determined by using an optically dilute method20 in which 9,l O-diphenylanthracene was employed as the standard. The picosecond fluorescence measurements were made on a mode-locked Nd:glass laser system. The pulse widths and the energies of the single pulses were 4 ps and 10 pJ, respectively. A single pulse was passed back and forth twice through the first amplifier and amplified to an energy of about 5 mJ. The single pass arrangement of the second amplifier created pulses with an energy of 100 mJ. The fundamental beam of 1054 nm was collimated and passed through a KDP crystal to generate the second harmonics of 527 nm. The third harmonics (351 nm) were generated by a second KDP crystal. Fluorescence decay was measured by a streak camera. The time profiles of the fluorescence were displayed by a temporal disperser (Hamamatsu C 1370). The streak image was then monitored by a TV camera and digitized by a temporal analyzer (Hamamatsu C 1098). The digital data were transferred to a microcomputer for further data treatment. The nanosecond transient absorption measurements were carried out by using a nitrogen laser (337.1 nm, 4 mJ/pulse, 7-11s pulse duration) as an excitation light source.16 A xenon flash lamp (Ushio UXL-150DS, 400-ps duration, 150 W) was fired synchronously with the nitrogen laser and it provided a probe light for the detection of the transients. After passing through a
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Figure 1. Schematic energy level diagrams for three typical types of rare-earth complexes. The lowest ff* levels of the central metal ion are located a t energetically lower (a) or higher (c) states than the lowest triplet level (T,) of a ligand. There are no ff* levels in La3+ and LU” complexes (b). Radiative, very weakly radiative, and nonradiative processes are represented by &, -, and m,respectively.
with inner 4f electrons in a lanthanide ion (Ln3+). The lanthanide ions of the rare-earth-metal elements are paramagnetic except for La3+ and Lu3+. Therefore, the rare-earth-metal ions are expected to influence significantly the relaxation processes of the ligands. Unfortunately, ligand localized phosphorescence with paramagnetic rare-earth metals has scarcely been observed due to the fast energy transfer to the metal ions, and the intensity of the ligand localized fluorescence is too weak to determine the fluorescence lifetimes (case a in Figure 1). So there has been little attention paid to the relaxation processes of the ligand localized states of the rare-earth complexes, In order to investigate the influence of a central metal ion, it is appropriate to use certain trivalent ions such as La3+,Gd3+and Lu3+which have no available energy levels for energy transfer from the ligand (cases b and c). Yuster and Weissman first noticed a remarkable metal effect by using samples of a complex of dibenzoylmethane derivatives and the trivalent ions of AI, Sc, Y , La, Gd, and L u . ~ In the paramagnetic Gd3+complex, they found a prominent feature that the phosphorescence lifetime decreases significantly and fluorescence was not observable in contrast to the appreciable fluorescence intensity of the other diamagnetic metal complexes. This effect caused by a Gd3+ion is the so-called “paramagnetic effect”, that is, singlet-triplet mixing between the electronic states of a ligand is enhanced by the inhomogeneous magnetic field caused by the paramagnetic ion. Since then, however, several authors have had misgivings about this paramagnetic effect.*-I0 Several researchers have substantiated the existence of the paramagnetic effect in and tetraphenylp~rphin’~ acetylacetone,” tetra-ptolylporphin,12-14 (7) Yuster, P.; Weissman, S. I. J. Chem. Phys. 1949, 17, 1182. (8) Tsubomura, H.; Mulliken, R. S. J . Am. Chem. SOC.1960, 82, 5966. (9) Evans, D. F. J . Chem. Sac. 1961, 1987. (10) Porter, G.; Wright, M. R. Discuss. Faraday Sac. 1959, 27, 18. (11) Crosby, G. A,; Watts, R. J.; Westlake, S.J. J . Chem. Phys. 1971, 55, 4667
(12) Tsvirko, M. P.; Stelmakh, G. F.; Pyatosin, V. E.; Solovyov, K. N.; Kachura, T. F. Chem. Phys. Lett. 1980, 73, 80. (13) Stelmakh, G. F.; Tsvirko, M. P. Opt. Spektrosk. 1980, 48. 185.
(14) Tsvirko, M. P.; Solovyov, K. N.; Stelmakh, G. F.; Pyatosin, V. E.; Kachura, T. F. Opt. Spekrrosk. 1981, 50, 5 5 5 . (15) Harriman, A. J. Chem. SOC.,Faraday Trans. 1 1980, 76, 1978. (16) Tobita, S.; Arakawa, M.; Tanaka, I. J . Phys. Chem. 1984, 88, 2697. (17) Whan, R. E.; Crosby, G.A. J. Mol. Spectrosc. 1962, 8, 315. (18) Lippert, E.; Nagele, W.; Seibold-Blankenstein, I.; Staiger, U.;Voss, W. Z. Anal. Chem. 1959, 17, 1. (19) Melhuish, W. H. Appl. Opt. 1975, 14, 26. (20) Demas, J. N.; Crosby, G.A. J . Phys. Chem. 1971, 75, 991.
The Journal of Physical Chemistry, Vola89, No. 26, 1985 5651
Ligand Localized Intersystem Crossing Wavelength 600
I
500
I nm
400
I nm
Wavelength 300
600
I
500
400
300
1.0VI
c
e
e
- 0.5-
c
VI
c
c
C I
0-
15
20 ~.
25
30
35
Wavenumber I k K Figure 2. Absorption, emission, and excitation spectra of Ln(MS)3 in EME at room temperature.
Wavenumber I kK Figure 3. Emission and excitation spectra of LII(MS)~in EME at 77 K.
monochromator (Nikon P250), the monitor light from the xenon flash lamp was detected by a photomultiplier tube (Hamamatsu R928) and the time profiles of transient absorption were displayed on an oscilloscope. The signals of the transient absorption in a single shot were stored by using a digital memory (Iwatsu DM901, 10 ns/channel) and then transferred to a microcomputer. The accumulated signals from the digital memory were analyzed by the microcomputer and the final spectra were recorded by a printer or an XY plotter.
TABLE I: Fluorescence Quantum Yields (+F), Fluorescence ( T ~ and ) ) at Room Temperature, and Phosphorescence ( T ~ Lifetimes Phosphorescence Lifetimes ( T p ” ) at 77 K @F T F , ns ~ p p, s T ~ ” ,ms
Results Absorption, Emission, and Excitation Spectra. Figure 2 shows the absorption, emission, and excitation spectra of Ln(MS), taken at room temperature. EME was used as the solvent because of the high solubility of the complexes in it and the glassy formation at 77 K. Since the absorption and excitation spectra are almost identical for the four complexes employed the absorption and excitation spectra for Lu(MS), only are shown in this figure. The absorption bands of the complexes are located around 350 nm. The ultraviolet absorption band around 310 nm is due to the presence of free methyl salicylate partially dissociated in the solvent. A similar phenomenon is also observed in rare earth complexes with o-hydroxybenzophenone.21 In the complexes prepared in this work, no intra-4f transition of the rare-earth ions (Gd3+ and Tb3+) was observed in the absorption spectra. This indicates that the absorption cross sections corresponding to these transitions are extremely small because of their forbidden prope r t ~ .The ~ emission spectra of the three complexes excepting Tb(MS), are broad and structureless. The emission spectra of La(MS)3 and L u ( M S ) ~contain only a ligand localized fluorescence band (410 nm), while that of Gd(MS), is composed of two bands. The weak band at around 410 nm is attributed to fluorescence and the strong one at around 490 nm, which has a long lifetime, is assigned to phosphorescence. On the other hand, the emission spectrum of Tb(MS), is peculiar compared to those observed for the other systems. Instead of the ligand localized emissions, five atomlike bands appear which can be assigned to 5D4 7FJ ( J = 2, 3, 4, 5 , 6) transitions of the Tb3+ ion. The intramolecular energy transfer from a ligand excited triplet state to the central metal ion is energetically possible only in the Tb(MS), system. The absence of the 490-nm emission band in Tb(MS), supports the view that the 490-nm band of Gd(MS)3 should be assigned to phosphorescence. Figure 3 shows the emission and excitation spectra of Ln(MS), taken at 77 K. The excitation spectrum of Lu(MS), is shown as representative for the same reasons as described earlier. Vibrational structures did not appear, although the samples were cooled to 77 K. However, it should be noted that phosphorescence is beginning to appear in the La(MS), and Lu(MS), complexes. Fluorescence Quantum Yields and Lifetimes. The fluorescence of Ln(MS), at room temperature were dequantum yields (aPF)
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(21) Crosby, G. A.; Whan, R. E.; Freeman, J. J. J . Phys. Chem. 1962,66, 2493.
~
La(MSL Gd(MSj3 Tb(MS), Lu(MS),
0.24
