J. Phys. Chem. 1!393,97, 13496-13499
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Highly Resolved Optical Spectra of Pd(2-thpy)z in a Shpol’skii Matrix H. Yersin,’ S. Schiitzenmeier, and H. Wiedenhofer Institut f i r Physikalische Chemie, Uniuersitiit Regensburg, 0-93040 Regensburg, Germany
A. von Zelewsky’ Institut de Chimie Inorganique et Analytique, Universitb de Fribourg, CH- 1700 Fribourg, Switzerland Received: May 12, 1993; In Final Form: September 28, 1993’
Pd(2-thpy)2 is a representative of the interesting new class of ortho-metalated compounds. For the first time, we present highly resolved emission and excitation spectra. This could be achieved by using the Shpol’skii matrix isolation technique. From the intensity distribution of the highly resolved vibrational satellite structures and the corresponding vibrational energies it is concluded that the excited triplet lying at 18 418 f 1 cm-l and the singlet ground state exhibit nearly the same force constants and equilibrium positions of the potential hypersurfaces. The type of the electronic transition is assigned as being ligand centered with a relatively small MLCT admixture. The zero-field splitting of the triplet could not be resolved (experimental resolution 1 cm-l), but a t T = 1.3 K the three sublevels emit independently with 71 = 155 f 20 ps, 7 2 = 200 ps, and 7 3 = 1200 f 100 ps, respectively. With increasing temperature and thus increasing spin-lattice relaxation the emission lifetime becomes monoexponential with ~ ( 4 . 2K) = 235 f 10 ps.
Introduction Transition metal complexes with organic ligands have found a vast attention during the past decades. This was not only motivated by the enormous chemical variability and by possible applications like solar energy conversion but also by the scientific challenge to develop a better understanding of the electronic and vibronic properties of these compounds. In this respect [Ru(bpy)s]z+ and other polypyridine complexeshave become famous (e.g. see ref 1). A related class of compounds possessing very promising photochemical and photophysical properties is given by cyclometalated complexes. One interesting representative is Pd(2-thpy)z (2-thpy- = ortho-C-deprotonated form of 2-(2thienyl)pyridine).z4 It is subject of this paper to present and to discuss for the first time highly resolved excitation and emission spectra of this class of compounds. These could be measured by applying the Shpol’skii matrix isolation technique. This method is well established for organic m~lecules.~ One usually dissolves a low concentration of guest molecules in an n-alkane. At low temperatures this solvent forms a polycrystalline matrix. Under the conditions that guests and hosts possess at least partial geometric similarities (key and hole rule) and that the guests are nearly planar, neutral and exhibit no or relatively small dipole moments, one may obtain lattice guest sites of minor distortion and thus highly resolved optical ~ p e c t r a . These ~ conditions are fulfilled for the title compound in n-octane matrices as is shown in this contribution. Experimenw Section Pd(2-thpy)~is prepared in analogy to the procedure described for Pt(2-th~y)z.~The Shpol’skii matrix was prepared by first dissolving Pd(2-thpy)~in an intermediate solvent (1,4-dioxane, Merck, for spectroscopy) which was subsequently diluted with n-octane (Fluka, puriss., p.a.; > 99.5%) in a ratio of 1 5 0 vol/vol. The final concentration of Pd(2-thpy)~was approximately M. The solution was filled into a quartz cuvette (diameter 2 mm) and immediatelycooled. For samples obtained with cooling rates of about a hundred degrees per minute, we obtained sharp spectraofonlyonedominatingsiteofguest complexes(seebelow). For measurementsof the emission spectra the sampleswere excited *Abstract published in Aduance ACS Abstracts, November 15, 1993.
0022-3654/93/2097-13496$04.00/0
with a nitrogen laser (Lxc= 337.1 nm, Lambda Physik M 1OOO) while for the excitation spectra we used a dye laser (Lambda Physik FL 2000; line half-width, 0.15 cm-l; dyes, coumarine 307 and 153) which was pumped by an excimer laser (RD EXC 150). The optical set up of the detection system is described in refs 6 and 7. Theemissionspectrumiscorrectedforthespectralresponse of the monochromator and the photomultiplier, while the excitation spectrum is not corrected for the efficiency of the exciting laser. The emission decay was registered with a fast multichannel analyzer with a minimum dwell timeof 5 ns/channel (CMTE 7885). The infrared spectrum of Pd(2-thpy)~was measured at room temperature in KBr and polyethylene pellets, respectively, on a Nicolet 60 SX FT-IR spectrometer (resolution 4 cm-I). Results and Discussion Figure 1 demonstrates the enormous increase of resolution of the Pd(2-thpy)~emission spectrum obtained with an n-octane Shpol’skii matrix compared to the one with the usually applied butyronitrileglass forming matrix (dotted line spectrum). Figure 2 reproduces the excitation spectrum of Pd(2-thpy)~in the same Shpol’skiimatrix. These highly resolved spectra (half-widths of the peaks =2 cm-I) allow immediately to specify a number of important properties of the investigated compound. Electronic Origin. The dominating peak at 18 418 i 1 cm-I (line 1) lies exactly at the same energy in the emission and excitation spectra. This transition is assigned to the electronic origin, i. e. the 0-0-transition between the lowest excited electronic state 11) and the ground state (0).Further arguments for this assignment are given below. (0)is commonly assigned to a singlet, while 11) represents a state being mainly of triplet character, as has already been concluded from the relatively long emission lifetime even at 77 K.*3 No indicationof an occurrenceof further electronic origins (of the same chromophore) near (0) (1) was found. Therefore we conclude that the observed electronicorigin is composed of the three transitions corresponding to the three triplet sublevels, which, however, are not resolved within the experimental resolution of 1 cm-I. A related situation is found for most organic compounds.* Moreover, many investigations with organic molecules show that at sufficiently low temperature closely lying triplet sublevels are not thermally equilibrated (slow spin-lattice relaxation) and that they emit independently ac-
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CQ 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13497
Optical Spectra of Pd(thpy)z
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620
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19 000 18 000 17 000 16000 3 Icm”1 15000 Figure 1. Emission spectrum of Pd(2-thpy)z in an n-octane Shpol’skii matrix (narrow line spectrum) and in butyronitrile (broad spectrum) at T = 1.3 K. The energies of the vibrational satellites are specified relative to line 1 at 18 418 cm-I. The structure at the high energy side of line 1 results from different sites and vanishes with a site-selective excitation into a vibrational line corresponding to state (1) (line 1 + 695 cm-I). Concentration of Pd(2-thpy)~EJ lW5 M. Note: For a better comparison the broad band spectrum is shifted by 200 cm-l to the red.
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20 000 19 500 19000 5 Icm-’I 18500 Figwe 2. Excitation spectrum of Pd(2-thpy)sin an n-octane Shpol’skii matrix at T = 1.3 K. The emission is detected at = 17 702 cm-l (line 1 716 cm-I). For further experimental details sec Figure 1.
cording to their specific occupation and decay timeses Indeed, a similar behavior is also found for Pd(24hpy)~. The sublevels manifest themselves by the different emission decay properties. At T = 1.3 K the decay (measured at 18 418 cm-l) is composed of a clearly observable short-lived component of 71= 155 f 20 p, a relatively long-lived one of 7 3 = 1200 f 100 ps, and of a third one which could not very accurately be determined ( 7 2 = 200 c(s). With temperature increase (e.& to T = 4.2 K) the spin-lattice relaxation becomes faster and results in a thermal equilibration between the three sublevels. This gives a monoexponential decay with a mean decay time, which may be expressed by the three low temperature decay times, r(mean) = 3( 1/ T I + 1/72 + 1 / ~ 3 ) - l , (e.g. see ref 8). The calculated value of 7 = 245 ps is in a very good agreement with the measured ‘T = 235 10 ps at T = 4.2 K. The decay is strictly exponentialfor six lifetimes. Vibrational SatelliteStructures. Both, the excitation and the emission spectra exhibit rich satellite structures. These are assigned to vibrational transitions. The correspondingvibrational
*
energies of the excited and the ground state, respectively, are determined relative to the electronic origin line 1 (Figures 1 and 2). The ground-state vibrational energies are compared in Table I to IR data of the complex and to vibrational energies of metal coordinated thpy (from ref 9 ) , respectively. The good correspondenceof these values confirms further the assignmentof line 1 as electronic origin. An obvious mirror symmetry between the emission and excitation spectra is not found, but a careful inspection of the peak energies and intensity distributions allows to correlate an appreciable number of vibrational energies of the ground state 10) to the ones of the excited state 11) (Table I). It is seen that the vibrational energiesof 11) areonly slightly smaller than those of IO). Thus, it is concluded that the force constants of these states are not very different, at least with respect to the correlated modes. Moreover, the vibrational satellite structures do not show any dominating vibrational Franck-Condon progression as might erroneously be deduced from the broad band spectrum (brokenline spectrum,Figure 1). However, theemission
Yersin et al.
13498 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
TABLE I: Vibrational Satellites (cm-l) of Pd(2-thpy)z from Emission and Excitation Spectra Compared to IR Data (vs = Very Strong, s = Strong, m = Medium, w = Weak) emission at 1.3 K
excitatation at 1.3 K
0-0 (TI So) (18 418 k 1 cm-I)
0-0 (So TI) (18 418 f 1 cm-l)
+
32 m 50 w 79 m 111 w 150 m 196 w 211 m 229 w 261 w 284 m 292 m 364 s 376 s 424 w 447 s 451 w 528 m
IR 300 K
lattice modes' lattice modes'
32 s 52 w 73 m 107 w 148 m 208 m
assignments electronic origins
215 w 229 s 265 s
M-Lb (226') M-L (237') M-L (268') M-L (299')
359 m
M-L M-L M-L M-L
290 m 382 s 446 s
(376') (383') (435') (458')
455 w 528 w 575 m 592 m
625 s 643 m 650 s 713 w 716vs 874 w
695 s
623 w
63od
713 w 720 m 873 w
71 Id
930 m 989 s 1021 w 1025 m 1076 w
988d 1017 w 1086 s
1096 w 1153 w 1162 w
1275 m 1286 w 1299 w 1366 w
ll50w
1102m 1155 s 716
1206 s 1237 m 1267 m 1276 m
1276 m 1296 m
1301d 716 650 2 X 695
1398 m
1399d 2 X 716
1387 w 1392 m 1398 s 1427 w 1461 w 1468 w 1488 vs
1397 w
1485 s 1557 s 1590 m 1775 w
1991 w
2113m 2202 m 2476 w 2881 w 2971 w
+
1464 w
1607 m 1704 m 1850 m 1865 w 1936 w
+ 447
1564 m 1596 s
+
716 990 1485 + 290 1488 + 364 1488 376 1488 + 447
+
716
+ 1276
716 + 1398 1488 + 716 1488 990 1488 + 1398 2 X 1488
+
a These modes are also found to couple to a number of vibrational satellites. b These modes possess a significant metal-ligand (M-L) character. c Vibrational energies of Pt(2-thpy)214given for comparison. Ligand vibrations, data from ref 9.
spectrum reveals that for the prominent vibrational modes of 7 16 and 1488 cm-1 (695 and 1485 cm-1 in excitation) one finds also the 0-2 members of the corresponding progressions (Figure 1, Table I). But the intensities of the 0-2 members compared to
the ones of the electronic origin and of the 0-1 members, respectively, are very weak. From this it follows (e.g. see ref 1 1) that the excited state 11 ) and the ground state 10) have very similar equilibrium positions of the potential hypersurfaces. Quantitatively this may be expressed by the related Huang-Rhys factor" S,which is in all cases smaller than 0.2 (compared to the fictitious value S = 0.5, taken from the unresolved spectrum). A number of vibronic satellites does not exhibit any progression. These are assigned to Herzberg-Teller modes, which open radiative deactivation paths by vibronic coupling mechanisms.12J3 MLCT Character/Ligand-CenteredTransition. Up to now, it is not clear whether the discussed electronic transitions (0) (1) should be classified as ligand centered or whether an appreciable Pd4dE-thpy MLCT admixture is of importance. The intensity distribution of the vibrational satellites indicates a contribution of MLCT character, due to the Occurrence of a number of vibrational modes with metal-ligand character (Table I). These modes lie between =150 and 500 cm-1. They are differentiated from low-energy ligand modes by a comparison with the Pt(2thpy)l emission spectrum,14where the corresponding vibrational satellites are more intense and the vibrational energies are characteristically blue-shifted (compare with ref 10). For comparison, these vibrational energies of Pt(2-thpy)~are also given in Table I. The result, regarding a contribution of MLCT character, is in accordance with conclusions of Maestri, Balzani et al.293 drawn from a comparison of broad-band spectra of different related transition metal complexes. On the other hand, an important admixture of d-character to the discussed triplet should result in a marked zero-field splitting (zfs) of the sublevels according to a larger spin-orbit coupling. For example, a compound with an appreciable MLCT character like Pt(2-thpy)~ exhibits a total zfs of =15 cm-l l 4 while for typical MLCT compounds like [Ru(bpy)3I2+and [0s(bpy)3l2+one finds zfs of 60 cm-1 6 and 210 cm-l,I3 respectively. Since for Pd(2-thpy)z the upper limit of the zfs is 1 cm-I (experimental reso1ution)Is we conclude that the d-orbital contribution (or the MLCT character) to the discussed electronic stateis small. The relatively long lifetime of 1.2 ms for the long-living component supports this further. Consequently, we assign the lowest transition of the title compound to be mainly centered at the ligands with a relatively small d-orbital admixture.
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Conclusion The obtained highly resolved spectra of Pd(2-thpy)z allow a deeper insight into the electronic and vibrational structure of the compound. This could be achieved mainly by finding an adequate matrix, the Shpol'skii matrix. In particular, we were able to observe the origin corresponding to the lowest triplet, which exhibits a zero-field splitting of less than 1 cm-I. This and the intensity distribution of the resolved vibrational satellite structures allow to classify the type of the electronic transition as being ligand centered with a relatively small MLCT admixture. Moreover, it is concluded that the excited triplet and the singlet ground state exhibit nearly the same force constants and equilibrium positions of the potential hypersurfaces. On the basis of the obtainable high resolution, it will be possible to observe very accurately how alterations of the ligands and/or the central metal ion as well as the applicationof external fields will influence the electronic and vibrational structure. This may open new possibilities for a better defined chemical tuning. Acknowledgment. Financial funding of the 'Deutsche Forschungsgemeinschaft" and the 'Verband der Chemischen Industrie" are gratefully acknowledged. We also thank Prof. D. Schweitzer (Universitlt Stuttgart) for giving us the opportunity to measure ODMR spectra.
Optical Spectra of Pd(thpy)2
References and Notes (1 ) Ycrsin, H.; Braun, D.; Hensler, G.; Gallhuber, E.In VibronicProcesses inlwganic Chemistry;Flint,C. D.,Ed.;Kluwer: Dordrecht,TheNetherlands, 1989; p 195. (2) Balzani, V.; Maestri, M.; Melandri, A.; S a n d h i , D.; Chassot, L.; Cornioley-Dcuschel, C.; Jolliet, P.; Maeder, U.;von Zelewsky. A. In Photochemistry and Phorophysics of Coordination Compounds; Yersin, H . , Vogler, A., Eds.;Springer Vcrlag: Berlin, 1987; p 71. (3) Maestri, M.; S a n d h i , D.; Balzani, V.; von Zelewsky, A.; Jolliet, P. Helv. Chim. Acta 1988, 71, 134. (4) Chassot, L.; von Zelewsky, A. Inorg. Chem. 1987, 26, 2814. (5) Shpol’skii, E. V. Sou. Phys. Vsp. (Engl. Transl.) 1960, 3, 372. (6) Yersin, H.; Braun, D. Chem. Phys. Lorr. 1991, 179. 85. (7) Stock, M.; Yersin, H. Chem. Phys. Lert. 1976, 40, 423.
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13499 (8) Tinti, D. S.;El-Sayed, M. A. J . Chem. Phys. 1971, 54, 2529. (9) Zilian, A,; Giidcl, H. U. Inorg. Chem. 1992, 31, 830.
(IO) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wilcy-Interscience: New York, 1978. (11) Denning, R. G. In Vibronic Processes in Inorganic Chemistry, Flint, C. D., Ed.; Kluwer: Dordrecht, The Netherlands, 1989; p 111. (12) Fischer, G. In Vibronic Processes in Inorganic Chemistry; Flint, C. D., Ed.; Kluwcr: Dordrecht, The Netherlands, 1989; p 103. (13) Braun, D.; Hensler, G.; Gallhuber, E.; Yersin, H. J . Phys. Chem. 1991, 95, 1067. (14) Schiitzenmeier, S.Thesis, Universitit Regensburg, 1992. (15) Veryrecently, wecouldalsodetectODMRresonances, whichindicate a zfs of the order of 0.1 cm-l (Note Added in Proof).
