1372
J. Phys. Chem. 1985,89, 1372-1374
Sublevel Phosphorescence Spectra of Potassium Tetrakls(p-dlphosphonat0 )diplatlnate( I I ) Yuko Shimizu, Yuki Tanaka, and Tohru Azumi* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: October 5, 1984; In Final Form: December 7, 1984)
The phosphorescence spectra from the individual triplet spin sublevels of &[Pt2(P20SH2)4]were obtained. The spectrum from the upper degenerate component (E,representation in the D',,, double group) is well structured and has a long progression due to the Pt-Pt stretching vibration. The spectrum from the lower component (Al, representation in the LY4* double group) is, on the other hand, broad and structureless and is shifted, compared with the E, spectrum, to the red by -300 cm-I. This shift is attributed to the -50-cm-I zero field splitting together with a -250-cm-' nontotally symmetric vibration that is responsible for vibronic coupling.
observing only the thermal average of the two sublevel phosphorescence spectra. In this regard, we believe it to be of utmost importance to observe the phosphorescence spectra from the individual sublevels. In the case of organic molecules, the sublevel phosphorescence spectra can be obtained by utilizing the phosphorescence microwave double resonance m e t h ~ d . ~ -This ~ technique, however, cannot be applied in the present case because the zero field splitting is three orders of magnitude larger than the energies corresponding to the microwave frequencies. In this paper, therefore, we attempt to separate the two sublevel phosphorescence spectra- by timeresolved spectroscopy. Since the lifetimes of the two sublevels differ appreciably, the sublevel spectra may be separated out by time-resolved spectroscopy provided spin polarization is achieved. The essential prerequisite for this method is that the spin-lattice relaxation process should be sufficiently slower than the phosphorescence decay. Markert et a1.5 pointed out that this condition was not fulfilled a t all temperatures down to 1.6 K. We found, however, that the spin-lattice relaxation is somewhat suppressed even at -10 K and that nearly complete spin polarization is achieved at 1.3 K.
Introduction Potassium tetrakis(p-diphosphonato)diplatinate(II), K4[Pt2(P205H2)4],is a novel binuclear complex which exhibits intense phosphorescence even a t temperatures as high as -300 K. Spectroscopic properties of the complex have been studied by Fordyce, Brumer, and Crosby,l Che, Butler, and Gray? and Parker and C r o s b ~ . The ~ phosphorescence has been attributed to the emission from the 3A2ustate that is due to the 6p, 5d,z excitation. This assignment has been supported by our theoretical analysis4 which takes into account spin-orbit mixing among all 6p, clr 5d configurations. The phosphorescence lifetime vastly changes with temperature: e.g., 5.5 c at 300 K and 700 ps at 10 K. This marked temperature dependence has been interpreted by Fordyce et al.' in terms of two (one degenerate and one nondegenerate) Boltzmann populated triplet sublevels. Similar analysis has been performed by Markert, Clements, Corson, and Nagle.5 Both sets of authors concluded double that the degenerate sublevel (E,,representation in the group) is located above the nondegenerate sublevel (A,, representation in the tY4, double group) and that the energy gap is approximately 50 cm-'. That the AI, sublevel is located below the E,, sublevel has been shown by us4 to be generally true for any binuclear complex of tetragonal symmetry. We have analyzed the temperature dependence of the phosphorescence lifetime for a crystalline sample in a similar manner, and the results are essentially identical with those of Fordyce et al.' and Markert et alS5(vide infra). In this manner, the remarkable temperature dependence of the phosphorescence lifetime has been reasonably well understood. In contrast to this, however, the fact that the phosphorescence spectrum itself shows rather peculiar temperature dependence has not been discussed in the literature. The phosphorescence spectra of a crystalline sample observed at three different temperatures are shown in Figure 1. The broad and unstructured spectrum observed at room temperature becomes somewhat structured at 77 K as is expected, and the spectral location remains almost the same. Anomalous temperature dependence is, however, observed as the temperature is decreased to 4.2 K. The 4.2 K spectrum is red shifted by as much as -300 cm-', and, more peculiarly, the vibrational structure, somewhat resolved at 77 K, is completely lost. This anomalous behavior was already pointed out by Fordyce et al.' but no interpretation has been given. It appears that the difficulty in understanding the spectral behavior outlined above is largely due to the fact that one is ( i
Experimental Section Materials. K4[Pt2(PZOSH2)4] was prepared in a manner described in the literaturee2 The sample was finally recrystallized from a water-than01 mixture, and the yellow crystals thus obtained were used for the experiments. Apparatus. The excitation was carried out either by a highpressure mercury lamp, a nitrogen laser (Molectron UV400), or a nitrogen laser pumped dye laser (Molectron UV24 and DL14P). Phosphorescence was dispersed by a Spex 1702 monochormator equipped with an EM1 62568 photomultiplier tube. The timeresolved spectra were obtained by a PAR 162 boxcar integrator with two PAR 165 plug-ins. The decay measurements were carried out with an Iwatsu DM901 digital memory (IO-ns time resolution), and the data were transferred to a microcomputer for signal averaging. Results and Discussion Decays of the Phosphorescence. The phosphorescence decays were measured at various temperatures between -300 and 1.3 K. At temperatures above -10 K the decay is always single exponential. The analysis of the temperature dependence of the lifetime yields lifetimes for the A,, and E, sublevels of 5.5 ms and 2.8 ps, respectively, and an energy gap of 45 cm-'. The results are essentially identical with those reported At temperatures below 4.2 K, the decay behavior is somewhat complicated. At sufficiently long times the decay is a single
(1) Fordyce, W. A.; Brummer, J. G.; Crosby, G. A. J . Am. Chem. SOC. 1981, 103,7061. (2) Che, C. M.; Butler, L. G.;Gray, H. B. J . Am. Chem. Soc. 1981, 103,
._".
1'146 .
(6) Yamauchi, S.; Azumi, T. J. Chem. Phys. 1977,67,7. 1978,68,4138. (7) Murao, T.; Azumi, T. J . Chem. Phys. 1979,70,4460. 1980,72,4401. ( 8 ) Kokai, F.; Azumi, T. J . Chem. Phys. 1981,75, 1069. 1982,77,2757. (9) Asano, K.; Aita, S.; Azumi, T. J . Phys. Chem. 1983,87, 3829.
(3) Parker, W. L.; Crosby, G. A. Chem. Phys. Leu. 1984, 105, 544. (4) Shimizu, Y.; Tanaka, Y.; Azumi, T.J. Phys. Chem. 1984,88,2423. (5) Markert, J. T.; Clements, D.P.;Corson, M.R.;Nagle, J. K.Chem. Phys. Lett. 1983, 97, 175.
0022-365418512089-1372%01.50/0 0 1985 American Chemical Societv , , I
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Phosphorescence Spectra of K4[Pt2(P205H2)4]
The Journal of Physical Chemistry, Vol. 89, No. 8, 1985 1373
'
"t 0
Figure 1. The phosphorescence spectra of a K,[Pt2(P2OSH2).,]crystal at 300,77,and 4.2 K. The excitation was carried out by a mercury lamp.
The ordinate is calibrated to relative quanta. The anomalously large red shift and the loss of vibrational structures for the 4.2 K spectrum need to be elucidated.
IO0 TI M E / p
50
I50
200
Figure 3. The decay of the phosphorescence observed at 1.3 K. The excitation was carried out by a nitrogen laser. Different from the experiment shown in Figure 2, the intensity of the laser light was considerably reduced. The decay is approximately expressed as the sum of two exponential decays whose lifetimes are 3.6 ps and 5.5 ms.
20
(a)
w \
m IO
2
2
4 6 8 TIME/ms
0
101
i
r'
1
2
3
480
4
exp(klt)
Figure 2. The decay of the phosphorescence observed at 1.3 K. The
excitation was carried out by a nitrogen laser, and no effort was devoted to decrease the intensity of,the laser light. (a) Plot of the logarithm of the intensity (I) vs. time. Large deviation from exponentiality is observed. (b) Plot of reciprocal intensity vs. exp(klt), where k, represents the unimolecular decay rate constant, as determined (5.5 ms)-' from the long-time region of the decay. exponential with a 5.5-ms lifetime (Figure 2a). As one goes to shorter time regions the deviation from the single exponentiality is more pronounced. An effort to decompose the decays into several exponential decays was unsuccessful. The decay was satisfactorily interpreted by incorporating bimolecular quenching. In this mechanism, the population of the triplet excited state, n, is expressed as -dn/dt = kln
+ k2n2
(1)
The solution of eq 1 is =
(k + 2)
exp(k,t) - k2 k,
where no is the initial population. The plots of reciprocal intensity vs. exp(klt) were always found to be linear with a negative intercept; one of examples is shown in Figure 2. Thus the validity of eq 1 is substantiated. That the bimolecular decay process is pronounced only at low temperatures is obviously due to the fact that the measured first-order decay rate constant kl decreases at low temperatures. (Note that kl is the thermal average of the
50 0
520 540 WAVELENGTH / nm
560
Figure 4. The time-resolved phosphorescence spectra observed at 1.3 K. The excitation was carried out to the 'Azustate with a nitrogen laser. The solid and dotted durves represent respectively the 1-ps and 2-ms spectra after excitation. The 2-ms spectrum corresponds to the pure AI, sublevel spectrum. The 1-ps spectrum, on the other hand, has contributions from the two sublevels.
two rate constants associated with the sublevels.) As a further test of eq 1, we examined the effect of the intensity of the exciting light. As the intensity of the exciting light is weakened, the exponentiality of the decay is extended to shorter time regions. A decay obtained with fairly low exciting intensity is shown in Figure 3. Even in this case there remains a slight contribution from bimolecular quenching. The decay is nonetheless approximately expressed as the sum of two exponential decays whose lifetimes are 5.5 ms and 3.6 ps. These lifetimes nearly agree with the lifetimes of the two sublevels determined from the temperature dependence of the decay studied in the 10-300 K temperature range. The nonexponential decay behavior at temperatures below 10 K was already pointed out by Fordyce et al.' This finding, however, disagrees with the statement by Markert et al.5 The source of this discrepancy is unknown at present. At any rate, however, this experimental evidence clearly demonstrates that the spin-lattice relaxation processes are sufficiently slower than the phosphorescence decays at least for the crystals used in this experiment. Thus, the most fundamental prerequisite to observe the sublevel phosphorescence spectra is fulfilled. Time-Resolved Spectra Observed at 1.3 K. In Figure 4 are shown the time-resolved spectra observed at 1 ps and 2 ms after
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1374 The Journal of Physical Chemistry, Vol. 89, No. 8,
. 500
5 20 540 560 WAVELENGTH /nm Figure 5. The sublevel phosphorescence spectra of a K4[Pt2(P205H2)4] crystal observed at 1.3 K. Solid curve represents the E, sublevel spectrum. The spectrum was obtained by time-resolved spectroscopy at 1 ps after direct excitation to the 3A2ustate by 460-nm light from a nitrogen laser pumped dye laser. The dotted curve represents the AI, sublevel spectrum. This is the reproduction of the 2-ms time-resolved spectrum shown in Figure 4. 480
the excitation. The excitation was carried out to the singlet excited state, ,A2,, by a nitrogen laser. The 2-ms spectrum is broad and structureless and is essentially identical with the steady-state spectrum obtained by the mercury lamp excitation. The 1-ps spectrum is, on the other hand, observed at the shorter wavelength side, and, interestingly, the vibrational structure is somewhat resolved. At 2 ms after the excitation, the short-lived E, sublevel should be completely decayed away. Consequently, the 2-ms spectrum shown in Figure 4 should be regarded as the spectrum of the long-lived A,, sublevel. In the 1-ps spectrum, on the other hand, the E,, components should be predominant. Inspection of the decay shown in Figure 3, however, reveals that in the 1-ps spectrum the contribution from the A,, sublevel amounts approximately to 10% of the total intensity. In order to obtain the E, spectrum without interference from the A,, component, one needs to devise a way to enrich the population of the E,, sublevel. This is achieved by directly exciting to the triplet state, since only the E, sublevel is connected to the ground state by an allowed transition. In Figure 5 (solid curve) is shown the spectrum obtained by excitation to the triplet state with 460-nm light of a dye laser. The spectral location is roughly identical with that of the 1-ps spectrum shown in Figure 4. A remarkable difference is that the vibrational structures due to the 130-cm-' Pt-Pt stretching mode are now more clearly resolved.
Shimizu et al. In view of the expected population of the two sublevels, the spectrum thus obtained is regarded to represent the pure E, spectrum. For convenience of comparison, the A,, spectrum obtained above is also shown in Figure 5 (dotted curve). Why the SI-excited 1-ps spectrum (Figure 4) is not as well resolved as the TI-excited 1-ps spectrum is puzzling. As is discussed above, the former spectrum contains the contribution from the broad A,, spectrum to a certain extent. Efforts to deconvolute the observed SI-excited 1-ps spectrum into the A,, and E,, spectra, however, were not successful. This subject needs further study, but we tentatively interpret that the excitation to SIyields more than one emitting state and that the overlapping of several slightly shifted spectra makes the observed spectrum somewhat blurred. As is clear in Figure 5 , the A,, spectrum is red shifted relative to the E, spectrum. The exact amount of the shift between a broad spectrum and a structured one is hard to determine, but it is as large as -300 cm-]. The origin of the shift may be understood in the following way. The purely electronic transition from the A,, sublevel is forbidden and may be allowed only through the vibronic coupling involving either an eBor a2*vibrational mode. In view of the -50-cm-' zero field splitting, we suppose that vibrations of -250 cm-I are responsible for the vibronic coupling. Raman spectroscopyi0 reveals that there is an ee vibrational mode of 232 cm-I, and we tentatively consider that this is the vibrational species which contributes most effectively. We are not quite sure, at this moment, however, whether or not other vibrational species are also responsible. In fact, the broadness of the A,, spectrum appears to suggest that some other low-frequency phonon modes are also responsible. At any rate this subject needs further study. Once the two sublevel phosphorescence spectra are determined as is shown in Figure 5, the "anomalousn temperature dependence of the spectrum discussed in the Introduction is now well understood. In view of the expected radiative rate constant ratio (Markert et a1.j suggested a E,/A,, ratio of -5000) and the Boltzmann population, one may infer that at 300 K, and even at 77 K, one observes only the E, spectrum, whereas at 4.2 K one observes only the A,, spectrum. Namely, the spectral change observed in the 77-4.2 K temperature range (viz. large red shift and loss of structure) is not due to the temperature dependence of a single state. The 77 and 4.2 K spectra in fact represent two different sublevel phosphorescence spectra. Acknowledgment. We thank Professor T. Nakajima and Professor Y . Sasaki of this Department for stimulating discussion. We also thank Mr. T. Takano for hardware and software he has made to data transfer from the digital memory to a microcomputer. Registry No. K4[Pt2(P205H2)4], 79716-40-8. (10) Stein P.; Dickson, M. K.; Roundhill, D. M. J . Am. Chem. SOC.1983, 105, 3489.
