J. Phys. Chem. 1985,89, 4170-4171
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Photophysics of Dinuclear Rhodium(I)Isocyanides: Intersystem Crossing from an Upper Excited State of Rh2bd2+ Steven J. Milder* and David S. Kliger Division of Natural Sciences, University of California, Santa Cruz, California 95064 (Received: July 24, 1985)
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Rh2b4'+ (b = 1,3-diisocyanopropane) exhibits two emissions: a short-lived fluorescence (IA2, 'Alg) and a longer lived phosphorescence (3A2, IAlg). While the phosphorescence excitation spectrum follows the absorption spectrum, the fluorescence excitation spectrum intensity falls off at higher energies. This implies that intersystem crossing from an upper excited state kinetically competes with internal conversion. If one makes the reasonable assumption that the upper state intersystem crossing occurs from the 'E, state to the 3E, state, the rate of upper state intersystem crossing is estimated to be at least l O I 3 s-'.
Introduction The photophysical properties of the lowest singlet and triplet states of ligand-bridged dinuclear d8-d8 metal complexes have received intense study.'-" For dinuclear Rh(1) isonitriles it has been found that the two lowest energy strong absorption bands are widely separated in energy. In Rh2b42+(b = 1,3-diisocyanopropane) in CH3CN the two band maxima lie at 553 nm (e = 14500 M-I cm-I, IA,, 'A2,) and 318 nm (t = 31 450 M-' cm-I, IA,, 'E,,).9For Rh2b42+,emission has been reported from both the lowest excited singlet state (fluorescence) and the lowest triplet state (phosphorescence).2,6 Upon excitation at 553 nm, Rh2b42+in CH3CN at 20 OC exhibits fluorescence with a maximum at 656 nm, a quantum yield of 0.07, and a lifetime of 1.3 ns.'.l2 Under the same conditions a phosphorescence is seen with a maximum at 825 nm, a quantum yield of 0.32, and a lifetime of 8.5 P S . ~ We report here the wavelength dependence of the photophysical behavior of Rh2b42+. A dramatic decrease in the yield of fluorescence relative to the yield of the phosphorescence is seen when excitation is into the near-ultraviolet-absorbing 'E, state. This implies that intersystem crossing from the 'E, or some other state higher in energy than the lowest excited singlet state kinetically competes with internal conversion to the lowest excited singlet state ('A2"). This result is similar to observations previously reported for a few compounds which exhibit a decrease in the quantum yield of fluorescence relative to the quantum yield of phosphorescence as excitation energies increase.13-19 Such a
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(1) Miskowski, V. M.; Nobinger, G. L.; Kliger, D. S.; Hammond, G. S.; Lewis. N. S.: Mann. K. R.: Grav. H. B. J. Am. Chem. SOC.1978.100.485. ( 2 ) Milder, S. J.; Goldbeck, R: A.; Kliger, D. S.; Gray, H. B. J . Am. Chem. SOC.1980, 102, 6761. (3) Rice, S. F.; Gray, H. B. J . Am. Chem. SOC.1981, 103, 1593. (4) Dallinger, R. F.:Miskowski, V. M.; Gray, H. B.; Woodruff, W. H. J . Am. Chem. SOC.1981, 103, 1595. (5) Fordyce, W. A,; Brummer, J. G.; Crosby, G . A. J . Am. Chem. SOC. 1981, 103, 7061. (6) Rice, S.F.; Milder, S. J.; Gray, H. B.; Goldbeck, R. A,; Kliger, D. S . Coord. Chem. Reu. 1982, 43. 349. (7) Fordyce, W. A,; Crosby, G. A. J. Am. Chem. SOC.1982, 104, 985. (8) Rice, S. F.; Gray, H. B.J. Am. Chem. SOC.1983, 105, 457. (9) Mann, K.R.; Thich, J. A,; Bell, R. A,; Coyle, C. L.; Gray, H. B. Inorg. Chem. 1980, 19, 2462. (10) Marshall, J. L.; Stobart, S. R.; Gray, H. B. J . Am. Chem. SOC.1984, 106, 3027. (1 1) Milder, S. J. Inorg. Chem., in press. (12) Milder, S.J.; Kliger, D. S.; Butler, L. S.; Gray, H. B., manuscript in
preparation. (13) Hochstrasser, R. M.; Marchetti, A. P. J . Chem. Phys. 1%9,50, 1727. (14) Clark, S. E.; Tinti, D.S. Chem. Phys. 1980, 53, 403. (15) Kokal, F.; Azumi, T. J. Chem. Phys. 1982, 86, 177. (16) Li, Y. H.; Lim, E. C. J. Chem. Phys. 1972, 56, 1004. (17) Khalil, 0.S.; Seliskar, . J.; McGlynn, S . P. J . Chem. Phys. 1973, 58, 1607.
change in emission yields has been observed for NO2- salts in solid samples at cryogenic t e m p e r a t u r e ~ ' ~ -and l ~ for phthalazine,16 substituted anilines,17J8and some other aromatic organic molecules in low-temperature g1a~ses.l~ For many of the aromatic molecules in low-temperature glasses it is thought that this change in the relative emission yields of the fluorescence and the phosphorescence with excitation wavelength is due to the effect of the rigid matrix in freezing out a number of different conformations of the solute which have somewhat different emission properties."-I9 The current study avoids this complication as Rh2b2+is studied while dissolved in a fluid solution.
Experimental Section (Rh2b4)[B(Ph)4]2 and (Rh2b4)[BF4I2were synthesized by published procedures' and were recrystallized at least twice. All solvents were of the highest purity commercially available, and standard purification techniques were used when necessary. Emission and excitation spectra were taken on a Spex Fluorolog 2 spectrometer with a single 0.22-m excitation monochromator and a double 0.22-m emission monochromator. Both monochromators were operated with 0.5-mm slits. Samples were excited with a 450-W xenon lamp, and excitation spectra were corrected with a rhodamine B quantum counter detected with a 1P28 photomultiplier. The transient excited-state absorptions and the sequential two-photon emission were taken on an apparatus described previously.12s20 Attempts at detection of the weak twophoton emission used a 1P28 photomultiplier placed next to the sample and at right angles to the exciting laser beam. The light was collected after passage through a series of near-ultraviolettransmitting filters which had an optical window between 320 and 390 nm.
Results and Discussion Figure 1 shows the excitation spectrum of Rh2b42+obtained with the emission monochromator set to collect either the fluorescence (660 nm) or the phosphorescence (800 nm). For ease of comparison, the two spectra have been scaled to have the same signal size at 553 nm, the visible absorption maximum. The phosphorescence excitation spectrum follows the absorption spectrum more closely than does the fluorescence excitation spectrum. For the phosphorescence, the ratio of the signal size at 318 nm relative to 553 nm is 2.1, similar to the ratio of the extinction coefficients for the absorption peaks (31 500 to 14500 M-' cm-I). This ratio is only about 0.3 for the fluorescence. Thus, absorption into the 'E, state does not lead to efficient population (18) Dubroca, C.; Lozano, P. Chem. Phys. Le??.1974, 24, 49. (19) Itoh, K.; Azumi, T. J . Chem. Phys. 1975, 62, 3431. (20) Milder, S. J.; Kliger, D. S.J . Am. Chem. Soc., in press.
0022-3654/85/2089-4170$01.50/0 0 1985 American Chemical Society
J . Pkys. Chem. 1985, 89, 4171-4173
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Figure 1. Excitation spectra of Rh2b?+ in CHJCN at 20 O C taken with the emission monochromator set to observe the phosphorescence ()A2, lAlg, 800 nm, - -) and the fluorescence (IA2" IAIg,660 nm, -). The spectra are corrected for variations in the excitation light intensity, and the displayed intensity is arbitrary. The spectra have been scaled to give the same intensity at the maximum of the visible absorption peak (553 nm) for convenience of display.
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of the fluorescent 'A2, state (& = 0.15), yet it does not appreciably decrease the quantum yield of the phosphorescence. This implies that there is a highly efficient intersystem crossing pathway somewhere above the relaxed 'A2, state. Indeed, in Rh2b42+it is reasonable to expect facile intersystem crossing from 'E,, to 'E,,, as these states have been shown to be strongly spin-orbit couThough it is less obvious why there would be such large spin-orbit coupling in these cases,it is possible that the intersystem crossing occurs from a ligand field state or a charge-transfer state which lies somewhere in energy between the 'E,state and the *A2, state. Rice and Gray' have shown in polarized absorption spectra of single crystals of (Rh2b4)[B(Ph)4]2that there are at least two weakly absorbing states whose energies lie between the two strongly absorbing states. The low internal conversion yield after excitation into the 'E, absorption has been confmed by transient absorption experiments. Excitation of Rh2b2+ with a 7-ns, 532-nm pulse from a Nd:YAG laser gives transient absorption which occurs during the laser pulse and which has been assigned as due to absorption from the lowest excited singlet state to upper singlet states ('Azu S,).12 This is followed by longer lived absorption which is due to absorption from the lowest triplet ('A2,) to higher triplet states.'v2 Upon excitation at 355 nm with the same optical geometry, optical density of sample at the exciting wavelength, and number of photons per pulse, the longer lived excited triplet-state absorption signal size is the same to within 10%. However, here no obvious
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excited singlet-state absorption is seen to occur during the laser pulse. From the lack of clear singlet excited-state absorption signal upon excitation at 355 nm, it can be estimated that the lAlu population is at least 5 times smaller that the population resulting from 532-nm excitation. This is further evidence that upper state excitation does not lead to efficient population of the lowest singlet while it does lead to efficient population of the lowest triplet. Attempts were made to observe emission directly from the upper excited states, 'E, and 3EU, by exciting directly into the 'E, state at 318 nm. However, the emission signal was impossible to differentiate from emission from ultraviolet-absorbing impurities and scattering from the sample and solvent. Thus, the 532-nm line from the Nd:YAG laser was used to initiate sequential two-photon absorption and subsequent emission. This technique has been shown to be more sensitive than direct one-photon absorption in the detection of weak emissions from upper excited state^.*^-^^ In the present case, Rh2b42+in CH3CN at 20 O C did not give emission between 320 and 390 nm which increased as the square of the laser power following excitation at 532 nm. Laser energies between 50 FJ/pulse and 5 mJ/pulse were used. From this and the lack of a clear signal upon direct excitation at 318 nm using a conventional fluorometer, the emission yield can conservatively be estimated to be less than This value, and the Strickler-Berg calculated lifetime, give an upper limit to the s. lifetime of the 'E, state of approximately Similar results as those discussed above were seen for the complex Rh2(TMB)42+(TMB = 2,5-dimethyl-2,5-diisocyanohexane) both in fluid solution and in a low-temperature glass. This, and similar work on isoelectronic Ir(1) complexes,25show that in dinuclear d8-d8complexes upper state intersystem crossing kinetically competes with internal conversion. Thus, the large spin-orbit coupling possible in transition-metal complexes can lead to very rapid intersystem crossing. It appears that the intersystem crossing rate varies by orders of magnitude within these complexes, depending on the state. For Rh2b:+ the intersystem crossing rate constant from the lowest singlet state (IA2,) is 8 X lo8 s-', the reciprocal of the fluorescence lifetime, while the intersystem crossing rate constant is probably greater than l O I 3 s-I for intersystem crossing from the 'E,upper excited state.
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(21) Isci, H.; Mason, W.
R. Inorg. Chem. 1975, 14, 913.
Acknowledgment. We thank Ms. Janet L. Marshall and Professor Harry B. Gray for useful discussions through the course of this work and the National Science Foundation for partial financial support (PMC83-17044). ~
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(22) Omer, G. C.; Topp, M. R. Chem. Phys. Lett. 1975, 36, 295. (23) Topp, M. R.;Lin, H.-L. Chem. Phys. Left. 1977, 47, 442. Chem. Phys. 1981,60, 47. (24) Topp, M. R.; Lin, H.-R. (25) Marshall, J. L., unpublished results.
Production of HOP*in the Track of High-Energy Carbon Ions' Jay A. LaVerne and Robert H. Schuler* Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: June 25, 1985)
The radiation chemical yields of HOy produced by irradiating water with I2C ions of 37-100-MeV initial energy have been determined. These data have been combined with previous values for lower energy carbon and helium ions to obtain the differential yields over the range 2-102 eV/A. The close similarity between the yields for helium and carbon ions of the same LET suggests that the reaction volume of interest extends somewhat beyond the track core, especially for helium. LET appears to be a useful parameter to describe the production of HOz. in the tracks of heavy particles.
Recently we have reported measurements on the radiation chemical production of H02. from water in the track of carbon
ions with energies up to 35 MeV.2 The differential H02. yield observed at 25 MeV, where the LET (linear energy transfer) for
0022-3654/85/2089-4171$01.50/00 1985 American Chemical Society
