4446
Inorg. Chem. 1991, 30, 4446-4452 Contribution from the Arthur Amos Noyes Laboratory (No. 8348), California Institute of Technology, Pasadena, California 9 1 125, and Bandgap Technology Corporation, Broomfield, Colorado 80021
Electronic Spectra and Photophysics of Platinum(I1) Complexes with a-Diimine Ligands. Solid-state Effects. 2. Metal-Metal Interaction in Double Salts and Linear Chains Vincent M. Miskowski*.la and Virginia H. Houlding*Slb Received November 27, 1990 Solid-state luminescence and absorption data are reported for linear-chain compounds of Pt(l1) complexes containing a-diimine ligands in combination with CN- and amine ligands. Both neutral (e.g. Pt(bpy)(CN),) and double-salt (e.g. [Pt(bpy),] [R(CN),]) compounds are examined and compared. All of these compounds have d(Pt,) near 3.3 A, and all exhibit strong electronic emission with A, = 560-715 nm and knd = 105-106 s-l. This emission is attributed to a do+(d,z(Pt)) **(a-diimine) triplet-parentage excited state. The assignment is based primarily upon the observed vibronic structure, but correlation with d(Pt,) is also considered. The factors involved in red-shifting this particular type of metal-to-ligand charge-transfer transition relative to the monomer are discussed, as are criteria for distinguishing metal-metal, charge-transfer, and ligand-localized electronic transitions in Pt(I1) a-diimine complexes.
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Introduction We recently reported a study2 of absorption and emission from Pt( 11) coordination complexes containing a-diimine ligands in environments where metal-metal interactions could be considered negligible. We found that truly "monomeric" Pt(I1) coordination complexes containing 2,2'-bipyridine (bpy) or 1,IO-phenanthroline (phen) exhibit just two types of lowest energy (emissive) excited states. If the complex contains weak-field ligands in addition to a diimine ligand (e.g. Pt(bpy)C12), the lowest energy excited state is a triplet ligand field state with a very broad red emission. If the complex contains only strong-field ligands (e.g. Pt(bpy)(en)2+ or Pt(bpy)22+),the lowest energy excited state is a slightly perturbed a-diimine intraligand (IL) '(r?r*) state with a highly vibronic structured green emission. Examples of metal-to-ligand charge transfer (MLCT) emission could not be found for this class of compounds, although some upper excited states could be identified as Pt-to-a-diimine MLCT. Solid-state electronic spectra of Pt(I1) complexes are often perturbed from those of the solution species. In our earlier work2 we identified one class of weak perturbation as involving "excimeric" interaction between the a-diimine ligands of two or more monomers, resulting in a broad red-shifted '(71-7r*) emission. Our prototypical example of this case was the compound [Pt(phen)2]C12;3H20, which has weak dimers (intermonomer spacing of 3.71 A) in the crystal. Pt( 11) a-diimine complexes often crystallize in linear-chain structures in which Pt-Pt distances are on the order of 3.0-3.5 A, leading to stronger perturbation. Examples of such materials include both neutral complexes such as Pt(bpy)X2 (X = C1,4 CNS) and double salts such as6 [Pt(bpy)J [Pt(CN),]. These materials are generally intensely colored, in contrast to the nearly colorless monomers, and their electronic emissions are likewise distinctive. We will examine several such materials in this paper and will endeavor to show how metal-metal interaction affects electronic absorption and emission spectra, most notably the Pt **(adiimine) MLCT transitions.
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Experimental Section The compounds [Pt(bpy)(en)](CIO,),, [Pt(phen)(en)](ClO,),, [Pt(phen),]CI2, and [Pt(bpy),](CIO,), were available from our previous (1) (a) California Institute of
Technology. (b) Bandgap Technology Corp. Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989, 28, 1529. Hazell, A.; Mukhopadhyay, A. Acta Crystallogr. 1980, 836, 1647. (4) (a) Textor, M.; Oswald, H. R. Z . Anorg. Allg. Chem. 1974,407, 244. (b) Osborn. R. S.;Rogers, D. J . Chem. Soc., Dalton Trans. 1974, 1002. (c) Bielli, E.;Gidney, P. M.; Gillard, R. D.; Heaton, B. T. J . Chem. Soc.. Dalton Trans. 1974, 2133. (5) Che, C.-M.; He, L.-Y.; Poon, C. K.; Mak, T. C . W. Inorg. Chem. 1989, 28, 308 1. (6) (a) Little, W. A,; Lorentz, R. Inorg. Chim. Acra 1976, 18, 273. (b) Kiernan, P. M.; Ludi, A. J . Chem. Soc.,Dalron Trans. 1978, 1127. (c) Houlding, V. H.; Frank, A. J. Inorg. Chem. 1985, 24, 3664. (2) (3)
study., The compounds Pt(bpy)(CN),, Pt(phen)(CN),, and Pt( Me2bpy)(CN),.2H20(where Me,bpy is 5,5'-dimethyl-2,2'-bipyridine) were prepared by the methods of ref 5 ; we thank Dr. C.-M. Che for donating samples of these compounds that were used in our initial studies. The double-salt compounds with Pt(CN),2- anions listed in Table I were prepared by coprecipitation from aqueous solutions according to literature method^.'^^^ These compounds usually precipitate as yellow hydrates which readily lose water upon washing with organic solvent, vacuum-drying, or, in some cases, simply standing in air, yielding the intensely colored (usually deep orange to red) anhydrous forms. The double salts [Pt(bpy),)[Pt(CN),], [Pt(phen),][Pt(CN),], and [Pt(phen)(en)][Pt(CN),] all formed hydrates. Satisfactory elemental analyses were obtained for the anhydrous forms; the hydrates analyzed well for relative values of C, N, and Pt but were not generally robust enough to yield reproducible values of hydration number. The previously unreported compound [R(bpy)(en)][Pt(CN),] was the only double salt that did not initially precipitate as a hydrate. It was prepared by slow addition of a stoichiometric amount of an aqueous solution of Na,Pt(CN), to slow addition of a stoichiometric amount of an aqueous solution of Na2Pt(CN), to a hot (60 OC stirred aqueous solution of [Pt(bpy)(en)](CIO,),. After the mixture was cooled to room temperature, the bright yellow precipitate was filtered off and washed with water, ethanol, and ether. Vacuum-drying overnight left the color unchanged, and an elemental analysis indicated an anhydrous formulation for the yellow solid. Anal. Calcd (found) for Pt2Ci6Hi6N8: c , 26.45 (26.18); H, 2.22 (2.37); N, 15.42 (15.43). Powder X-ray diffraction spectra of two representative double salts, [Pt(bpy)(en)][Pt(CN),] and anhydrous [Pt(bpy),][Pt(CN),], were obtained on a Phillips APD3600 diffractometer using Cu Ka radiation and were indexed by a combination of manual and iterative computer fitting. Transmission electron microscopy (TEM) was used to collect electron diffraction micrographs of single microcrystals (ca. 20 X 500 pm needles) of [Pt(bpy)(en)] [Pt(CN),] for comparison with the powder X-ray diffraction data. Hydrated salts were not stable to either of these techniques. Emission, absorption, and lifetime measurements in general employed the equipment and methods of ref 2. For measurements made on hydrated forms, samples were stored and examined as aqueous slurries or colloids. Emission lifetime measurements on hydrated samples were unsuccessful; intense coloration (indicative of dehydration) developed immediately upon exposure of samples to laser pulses. Several nanosecond time scale emission lifetime measurements were performed at The Center for Fast Kinetics, The University of Texas, Austin, TX. Results Monomers. In order to facilitate a discussion of the solid-state effects on the electronic structure of strongly interacting materials, it is useful to consider first the structure of the analogous monomeric Pt( 11) complexes. We have discussed the electronic absorption and emission spectra of several monomeric Pt(1I) a-diimine complexes in considerable detail in ref 2, including Pt(bpy)22+,Pt(phen)22+,and Pt(bpy)(en)z+. The reader is also referred to refs 7-10 for other recent work concerning the spectra (7) Maestri, M.; Sandrini, D.; Balzani, V.; von Zelewsky, A,; DeuschelCornioley, C.; Jolliet, P. Helu. Chim.Acfo 1988, 71, 103.
0020-1669/91/1330-4446$02.50/00 1991 American Chemical Society
Inorganic Chemistry, Vol. 30, NO.23, 1991 4447
Platinum(I1) a-Diimine Complexes Table I. Photophysical Properties of Linear-Chain Pt(I1) Compoundso
compd (powder color) [Wbpy)(en)l [Pt(CN),I (bright yellow) [Pt(phen)(en)l [Pt(CN),I (bright yellow) [ P t ( b ~ ~ )[Pt(CN)41*2Hz@' zl (yellow) [Pt(b~y)zI[Pt(CN)41 (orange) [Pt(phen)~l[Pt(CN),I (violet) Pt(bPY)(CN )2'XH20 (yellow) Pt(bPy)(CN)z (orange) Pt(phen)(CNh (red)
diffuse reflectance 330, 450
, A, nm emission excitation 500 (sh)
310, 430
500 (sh)
emission 560 (584, 30 K) 560
485
500 (sh)
330, 520
150
emission quantum yieldb 0.17 (0.30, 30 K) intenseC
570
a,; moreover, the interaction may be stronger in the excited state2 (excimeric). The lowest energy MLCT transition should therefore be b2(da*) a,(s*(bpy)), ‘Al IB2, which is dipole-allowed with z-polarization. The corresponding ’B, state has spin-orbit components of A,, B,, and A2 symmetries, the first two yielding dipole-allowed transitions with the ground state of respectively y- and x-polarization, that is, perpendicular to z. These predictions are all in accord with experiment. Note, moreover, that since these transitions transform identically with the b2 a, singlet and triplet da* (pa, ?r*(CN)) transitions, all of the extensive single-crystal experiments performed by Gliemann’s group21 that were shown to be consistent with the d-p assignment are actually equally consistent with our assignment. Consideration of M O schemes similar to Figure 5 for C2,and D2h stacking patterns (Figure 4) yields similar predictions; da* ?r*(bpy) MLCT is always predicted to be dipole-allowed parallel to z for the singlet-singlet transition and perpendicular to z for the singlet-triplet transition. There are two complications, however. First, the dipole-allowed MLCT transitions are always accompanied by Laporte-forbidden ones (since there are two combinations of the bpy A* orbitals; aS and b, for the c2h unit, b3gand b,, for the D2h unit), and there IS, in contrast to the case of the C , unit, no obvious way to predict their energetic ordering. Thus, gerade-symmetry MLCT states could conceivably end up as the lowest energy ones, which would result in a more complicated (vibronically allowed) emission behavior. A second complication is that, for the C2h stacking pattern, molecular zpolarization can in principle be mixed with y-polarization. However, the absorption data for Pt(en)CI, no such effect. We note that in a very recent paper36 Gliemann et al. have adopted an assignment scheme somewhat similar to our present one. The similarity is a rather formal one, since they label their LUMO as (using our notation) a (p,(Pt), r*(CN), ?r*(a-diimine)) orbital, thus begging the question of the best description of the electronic transition (and continuing to deemphasize the a-diimine contribution). Mixing among all these orbitals is, of course, symmetry-allowed and undoubtedly occurs to some extent. However, since the IL transitions of the a-diimine ligands are not greatly perturbed, we feel that it is reasonable to simply write the LUMO as a r*(a-diimine) orbital for the compounds of this study. We finally consider the interesting variation in the Huang-Rhys ratio S for the a-diimine effective vibration among the various compounds of this study (compare Figures 1 and 3). One possible
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(36) Biedermann. J.; Gliemann, G.; Klement, U.; Range, K.-J.; Zabel, M. Inorg. Chim.Acta 1990. 169, 63.
Miskowski and Houlding explanation we considered was that the C, stack structure of Pt(bpy)(CN), might affect S via bpy-bpy interactions. However, we have recently’’ determined emission spectra at 10 K for the compound Pt(bpy)CI2, which has the c2h stack structure with4 d(Pt2) = 3.40 A at room temperature. The spectral profile (maximum at 15 400 cm-I) is nearly identical with that of Pt(bpy)(CN),, with S for the 1400-cm-’ effective frequency being very similar. It may be noted that very large variations in S for the MLCT emissions of various Os(1I) and Ru(I1) bpy complexes have also been noted,Igband an explanation has been offered in terms of variations in the amount of mixing of the ligand ?rsymmetry levels with metal d orbitals as a function of charge and other ligands; a similar explanation may apply here.
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Conclusions The lowest energy excited states of isolated Pt(I1) complexes of the a-diimine ligands of this study are diimine ’IL states (except when weak-field ligands also in the complex drop a ligand field state below the 31L state),2 but a-diimine MLCT states lie not far to higher energy. In this connection, we note that ligands that are substantially better electron acceptors than the simple a-diimines of this study could conceivably make a ’MLCT state the lowest energy excited state, so these results are not necessarily generalizable. In particular, R(I1) complexes of orthometalated aromatics’ are not directly comparable, as these unusual ligands contribute their own complicated electronic structure to the picture and are not easily ranked among the more usual acidoamine and a-dimine ligands in terms of, e.g., electron-donating and/or -accepting ability. Metal-metal interaction in polynuclear linear-chain Pt(I1) compounds can greatly increase the energy of the da* combinations of metal d,z orbitals, which should clearly lower the energy of da* A* MLCT relative to monomer d 9 A* MLCT. We contend that this effect causes this type of MLCT state to be the lowest energy excited state in the linear-chain compounds of this study. For Pt-Pt distances that are longer than the 3.3-3.4 A of our compounds, the 31L state should eventually become the lowest energy excited state even for dimeric or linear-chain structures; this was our proposal2for the emission of the compound [Pt(phen)2]C12-3H20, and another example has been recently presented27cfor the compound Pt(bipyrimidine)(CN)2. A final interesting question is whether very short Pt-Pt distances might make a d p or d (p, ?r*(CN)) state the lowest energy excited state for an a-diimine complex. The phosphorescence of Pt(CN),” salts becomes roughly isoenergetic with that observed for Pt(bpy)(CN)2 when d(Pt2)becomes about 3.15 A.12 It must be remembered, however, that the MLCT states would also be strongly red-shifted for such a short d(Pt2), and ’MLCT would probably still be the lowest energy excited state. Acknowledgment. We thank C.-M. Che, C. Frito, and G. Gliemann for helpful comments and C.-M.C. in particular, for initially provoking our research on neutral-stack compounds by donating several samples. We are also grateful to Dr.Jordi Marti (Allied-Signal Inc.) for his work on the X-ray and TEM results. A preliminary version of this work was presented at the 8th ISPPCC, Santa Barbara, CA, August 1989.
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Registry No. Na2Pt(CN),, 15321 -27-4; [Pt(bpy)(en)] (CIO,),, 54806-37-0; [Pt(bpy)(en)] [Pt(CN),], 136503-94-1; [Pt(phen)(en)] [Pt(CN),], 136503-95-2; [Pt(bpy)i] [Pt(CN)4].2H20, 136503-96-3; [Pt(bpy)~][Pt(CN),], 54806-40-5; [Pt(phen),] [Pt(CN),], 59981-69-0; Pt(bpy)(CN)Z.xHzO, 136503-97-4; Pt(bpy)(CN)z. 54806-39-2; Pt(phen)(CN)z, 54806-38-1.
