J. Phys. Chem. 1988, 92, 6899-6901 Table V.35 We see that these shifts are significant (on the order of 5-10 cm-’) but not as large as might be expected considering the bond length changes. However, separate calculations have shown that even if the C(2)-C(4) and C(4)-C(6) internal coordinate force constants change as much as 20% due to bond length changes, the observed frequency shifts for the C=C “symmetric” and “antisymmetric” stretches are still of the order 5-50 cm-1.20 Conclusions The synthesis and crystal structures of RbPCP, KPCP, and PyHPCP have been reported. The PyH+ structure has a (possibly dynamic) disorder of the organic cation. The isomorphic KPCP and RbPCP structures have yielded information very complimentary to the previously reported structures, particularly of CsPCP. The minority population of the second configuration has been well accounted for. The structure of the first conformation shows that this versatile anion is very malleable. It readily adapts to different environments, particularly by CCN bending and stretching of the propene C=C bonds. Diffuse reflectance band edges were generally modeled quite well by EHT calculations but showed that factors other than anion (35) The splitting/shifting of the PCP IR bands in KBr or KC1 as noted in ref 15 and 17 may be due to trace amounts of water in the KBr as suggested in ref 15, but the present work suggests that ion exchange (forming KPCP) is a very plausible cause of this effect.
6899
geometry alone play an important role in determining the first transition energy. The new spectroscopic data from the KPCP and RbPCP samples afford a more complete analysis of the C=N frequencies, thus furthering the assignment of the in-plane fundamentals. The asymmetry of the C(2)-C(4) and C(4)-C(6) bond lengths produces a shift to higher frequency in the antisymmetric C=C stretching frequency and a downward shift in the symmetric C=C stretching frequency. Acknowledgment. Financial support from the National Science Foundation in the form of Grants DMR-8414566 and DMR8320556 is gratefully acknowledged. The X-ray diffraction laboratory was established through grants from N S F (Grant CHE-8408407) and The Boeing Co. We thank Traci Topping for assistance in preparing the figures. T.J.J. thanks the W.S.U. Graduate School for a summer fellowship. Supplementary Material Available: A table of least-squares planes calculations for the anion in the PyH, Rb, and K (two temperatures) structures, full atomic positions with anisotropic thermal parameters for the same four structures, unit cell drawings for the PyHPCP and RbPCP structures, a drawing of the PCP HOMO and LUMO, and a table of atomic positions and anisotropic thermal parameters for the CsPCP structure from ref 18 (12 pages) and tables of structure factors (34 pages). Ordering information is given on any current masthead page.
Vibrational Analysis of Sublevel Phosphorescence Spectra of Potassium Tetrakls(p-diphosphonato)diplatinate( I I). Mechanism of Radiative Transition for the Electronically Forbidden A,, Spectrum Takeshi Ikeyama,* Department of Chemistry, Miyagi University of Education, Aramaki Aoba, Sendai 980, Japan
Seiichi Yamamoto, and Tohru Azumi Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: January 21, 1988; In Final Form: May 31, 1988)
Well-structured phosphorescence spectra of the Kq[Pt2(P205H2)4].2H20 crystal are observed for both the A,, and E,, spin sublevels in contrast to the previously reported broad AI, spectrum. Vibrational analysis of the sublevel spectra and comparison with the excitation spectrum show the following properties: (1) In the E, spectrum the 0-0 transition is clearly observed. The structure of the spectrum is characteristic of a dipole-allowed transition. (2) In the Al, spectrum the 0-0 transition is rigorously absent. All the spectral features are built on a false origin. The dipole-forbidden A,, spectrum, therefore, is understood to obtain intensity completely by the vibronic coupling through a nontotally symmetric vibrational mode.
Introduction The phosphorescent state of the binuclear platinum complex K4[PtZ(P205H2)4] consists of two spin sublevels of the same The lifetimel~~.~ of one sublevel (the forbidden A,, state in D4h’ double group notation) is about 10’ times as long as that of the other sublevel (the allowed E, state). The emission spectra from these two sublevels have been obtained separately by the time-resolved meas~rements.~ However, the mechanism by which the forbidden Al, sublevel obtains radiative intensity has not yet been thoroughly understood. If the complex retains rigorous D4hsymmetry, the Al, sublevel has to obtain the radiative intensity only by the vibronic coupling (1) Fordyce, W. A.; Brummer, J. G.; Crosby, G. A. J . Am. Chem. SOC. 1981, 103, 7061. (2) Markert, J. T.; Clements, D. P.;Corson, M. R.; Nagle, J. K. Chem. Phys. Lett. 1983, 97, 175. (3) Parker, W.L.;Crosby, G. A. Chem. Phys. Lett. 1983, 105, 544. (4) Shimizu, Y.;Tanaka, Y.;Azumi, T. J . Phys. Chem. 1984,88,2423. ( 5 ) Shimizu, Y.;Tanaka, Y.; Azumi, T. J . Phys. Chem. 1985,89, 1372.
0022-3654/88/2092-6899$01.50/0
through nontotally symmetric vibrations. In this case the origins of the two sublevel spectra should differ by the amount corresponding to the sum of the zero-field splitting (-40 cm-1)1.235and the energy of the nontotally symmetric vibration responsible for the vibronic c o ~ p l i n g .If, ~ on the other hand, the symmetry of the complex is reduced in such a way that the A,, emission becomes allowed, the position of the origin of the AI, spectrum should differ from that of the E, spectrum only by -40 cm-I. As we have already reported,’ the band maximum of the A,, sublevel spectrum of K4[PtZ(P205H2)4] is shifted to the red as compared with that of the E,, spectrum; the amount of the shift (-300 cm-’) is much larger than the zero-field splitting (-40 cm-’). The large separation between the band maxima of the two sublevel spectra is also observed for the barium salt6*’of the same anion. These results appear to be consistent with the vibronic coupling m e ~ h a n i s m . ~ (6) Brummer, J. G.; Crosby, G. A. Chem. Phys. Lett. 1984, 112, 15. (7) Baer, L.; Gliemann, G. Chem. Phys. Lett. 1984, 108, 14.
0 1988 American Chemical Society
6900 The Journal of Physical Chemistry, Vol. 92, No. 24, 1988
1
Ikeyama et al. TABLE I: Energies of Origins and Vibrational Frequencies of Three Progressions in the E. Emission Spectrum at 1.3 K
EXC.
site I
I1 I11 IV
EO/cm-l
v1cm-l
20310 20 279 20 23 1 20310
113.3 114.0 117.4 113
TABLE E Energies of Origin Bands and Correspondisg Vibrational Frequencies of Three Progressions in the AI, Emission Spectrum at 4.2 K
site I
I1 111 IV I , ,
490
I
,
I
500 WAVELENGTH/nm
,
,
,
Enlcm-'
vlcm-'
2001 1 19 979 19 937 20010
115 114 116 116
?
510
Figure 1. E, emission and excitation spectra of the K,[R2(P20SH2),]. 2H20 single crystal. I, 11, and I11 indicate the site that the peak belongs to. u = 0, 1, 2, ... are the vibrational quantum numbers in the ground
state. In the excitation spectrum the origin band of site I11 is reversed because of the internal filter effect (Le., the excitation light is intensely absorbed at the surface of the single crystal).
However, an ambiguity remains. The E,, spectrum previously observed5 is reasonably structured, whereas the Al, spectrum is somewhat broad and unstructured. Owing to the difference in the spectral character (structured in one sublevel spectrum and unstructured in the other), an estimation of the amount of the shift between the two sublevel spectra has great uncertainty. For the purpose of clarifying the radiative character, it is necessary to determine the spectral origins and to make the vibrational analysis. Without such detailed vibrational analysis, it is uncertain whether the large spectral shift is due to the vibronic coupling, as is discussed above, or, alternatively, is due to the large shift in Franck-Condon maxima resulting from the large difference in the nuclear configurationsQ8 To remove the ambiguity, it is essential to obtain the wellstructured Al, spectrum in a similar manner as the E,,spectrum. The fact that the Al, spectrum was broad was probably due to the existence of many sites in the crystal. If this be the case, crystals of good quality might exhibit well-structured spectra. On the basis of this supposition, we carefully prepared the single crystal in a manner discussed by Rice and Gray,8 and with this crystal we were able to obtain well-structured sublevel spectra. In this paper, vibrational analysis is made to the newly observed sublevel spectra of the potassium salt. For the purpose of the vibrational analysis of the two sublevel spectra, a barium salt of the same anion might be expected to be adequate, because the relatively well structured E, spectrum has been reportedb8 for the crystal of the complex. However, our preliminary experiments show that the situation is much different in the case of barium salt; therefore, we restrict our discussion only to the potassium salt of the complex. The mechanism in the barium salt and its relation to the potassium salt will be discussed in a forthcoming paper.
Experimental Section Materials. K4[PtZ(P20SH2)4] was prepared in a manner described in the l i t e r a t ~ r e . ~Pure yellow single crystals of K4[PtZ(H2P205)4]~2H20 were grown in the absence of oxygen by the method of Rice and Gray.8 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 monochromator (8) Rice, S. F.; Gray, H. B. J . Am. Chem. SOC.1983, 105, 4571. ( 9 ) Che, C.-M.; Butler, L. G.;Gray, H. B. J . Am. Chem. SOC.1981, 103,
7796.
1,
,
-
fi,:
,
20
19
, 0bS: , , ,
, ,\
WAVENUMBER / I O 3 c m - 1
19 WAVENUMBER / I O 3 c m - l
20
18
Npre 2. Observed and calculated spectra of (a) E, and (b) AI, sublevels. The delay time used in the observation is 1-3 ps and 1.2-3.5 ms, respectively.
equipped with a Hamamatsu R928 photomultiplier tube. The time-resolved spectra were obtained either by a PAR 162 boxcar integrator with PAR 165 plug-in or by Becquerel-type sectors. Results and Discussion E , Spectrum. The onset region of the E, emission spectrum (observed at 1-3 p s after excitation) and that of the excitation spectrum are shown in Figure 1. Peaks in the El, emission spectrum can be analyzed in terms of the three different progressions indicated by I, 11, and I11 in this figure. The three vibrational progressions start from different origins. Moreover, at the same energies as the origins of E, emission, there exist distinct peaks in the excitation spectrum, as is also shown in Figure 1. The coincidence of the energies of the peaks in excitation and emission spectra indicates that these peaks are the 0-0 transitions of the individual sites. The energies of origins and vibrational frequencies of these sites (I, 11, and 111) are summarized in Table I. The vibrational interval is about 115 cm-l, which is assigned as the Pt-Pt stretching vibration.1-8*10 AI, Spectrum. The vibrationally structured Al, spectrum (delay time 1.2-3.5 ms) is shown in Figure 2. The fact that such a (10) Stein, P.; Dickson, M. K.; Roundhill, D. M. J . Am. Chem. Soc. 1983,
105. 3489.
Phosphorescence Spectra of K4[Ptz(PzO~H2)41.2H~0
The Journal of Physical Chemistry, Vol. 92, No. 24, 1988 6901
well-resolved spectrum was obtained is remarkable in view of the broad spectrum reported previously. The AI, spectrum also consists of three series of the progressions of the 115-cm-' Pt-Pt stretching mode. The energies of the spectral origins (Le., the shortest wavelength band in each spectrum) and vibrational frequencies of sites I, 11, and I11 are summarized in Table 11. Energy differences between the origins of E, (Table I) and AI, (Table 11) spectra in three sites are 299, 300, and 294 cm-', respectively. In view of the previously reported energy separation (41.9 cm-l)z between two sublevels, the electronic origin of the A', spectrum is proved to be absent. Vibronic Coupling Mechanism. The observed energy differences in origins of E, and A,, spectra should be the sum of the energy splitting AE between the two sublevels and the frequency of nontotally symmetric vibration which is responsible for the vibronic coupling. To determine the frequency of the nontotally symmetric vibrational mode, it is necessary to know the value of AE. For this purpose, we tried to observe the electronic origin of the forbidden Al, transition by the photon-counting method. This attempt was unsuccessful, however." We ought to be satisfied, therefore, to use the literature value2 of AE (41.9 cm-I), which is determined by the temperature dependence of the triplet lifetime. The vibrational frequency of the nontotally symmetric vibration is thus estimated to be 255 f 3 cm-l. In the Raman spectrum observed under the resonance conditionlo in solution the frequency 232 cm-' was the closest to the above estimate. This frequency, however, is not likely to correspond to the frequency of the coupling mode responsible for the AI, spectrum. It is further surmised that this band might be the overtone of the 115-cm-' totally symmetric band. Because of this doubt, we remeasured the Raman spectrum of the crystal under nonresonant conditions (using 632.8-nm laser excitation). We observed a peak at 248 cm-l, being approximately coincident with the estimated frequency of the vibronic coupling mode. A small difference between them may be attributed to the ambiguity in AE. (In the estimation of the energy gap from the temperature dependence of the lifetime, some ambiguity appears unavoidable.) Franck-Condon Analysis and Existence of a Broad Component. To obtain a better understanding of the spectral distribution, we attempted the Franck-Condon analysis of the sublevel spectra. We assumed the bandshape of each vibronic band on the basis of the Gaussian function. The Franck-Condon factors were calculated following the formula of Crosby et a1.12 Energies of
origin bands, vibrational frequencies (Tables I and 11), and bandwidths are determined directly from the observed spectra. The ratio (k2)of vibrational frequencies of excited state to the ground state, displacement ( b ) of the upper and lower potentials, and the relative intensity of each origin are the adjustable parameters. Attempts to simulate the observed spectra in terms of the three series of progressions (sites I, 11, and I11 in Tables I and 11) were unsuccessful, whatever the parameters we chose. For both spectra, the introduction of an additional broad component is necessary to explain the intensity distribution of overall spectra. The simulated spectra thus obtained are compared with the observed spectra in Figure 2a,b. The calculated spectra agree well with the observed one. The energies of the spectral origins and vibrational frequencies of the broad component in E, and Al, spectra are tabulated in the last row of Tables I and 11, respectively. The E, and Al, spectra are reproduced by a common set of parameters ( b = 3.5 and k2 = 1.05). The Franck-Condon analysis of the AI, spectrum is given for the first time. The calculated displacement for the structured component is 0.19 A. It nearly coincides with that reported* for barium salt (0.21 A). The value of k2 (= 1.10) indicates that the bonding character in the Pt-R bond increases in the excited state. Therefore, the displacement obtained above is expected to mean a contraction of the bond length, as was previously discussed.'** The bandwidth of the fourth component is 5 times as broad as the other structured components. The broadening is attributable to the inhomogeneous one. Namely, it is reasonable to expect that there are many sites which have different energies of origin band and different vibrational frequencies in a similar manner as the three sites which give rise to three distinct progressions. This component is expected to be responsible for the broad spectrum observed in the crude crystal. The vibrational structure of the broad component is not clear in the observed spectra; however, the result of Franck-Condon analysis of this component is remarkable. The separation between the spectral origins of broad components in E, and A', spectra is also -300 cm-' and supports the vibronic coupling mechanism which was presented directly in the case of the structured componen t . In conclusion, all (three structured and one broad) components of the Al, phosphorescence spectrum gain intensity through vibronic coupling. A mode in the Raman spectrum (248 cm-') is proposed as the coupling mode of e8 symmetry.
(1 1) Another series of very weak transitions were observed in the shorter wavelength E&n of the false ori@mofthe Ah spbcmm by a sellSitive measurement. These peaks are assigned as the progressions which start at E,, origins. Though the lifetime of the E,, state is about loo0 times shorter than that of the AI, state, a small population of the E,, state is expected to be thermally excited from the AI, state because the spin polarkation becomes imperfect at long time delay. (12) Hipps, K. W.; Merrelle, G. A.; Crosby, G. A. J . Phys. Chem. 1976, 80, 2232.
Acknowledgment. We want to express our thanks to Professor Ichiro Nakagawa and Professor Yoshiyuki Morioka of this department for their help in observation of the Raman spectrum. The present paper was partially supported by Grant-in-Aid for Special Project Research No. 6221 3003 and also by Grant-in-Aid for Research No. 62430001.
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RWtry NO. &[Pt2(P20sH2)4], 79716-40-8.
