Volume 104. number 4
CHEhllCAL PHYSICS LETTERS
10 February
1984
COMMENT POLARISATION RATIOS IN RUTHENIUM(H) COMPLEXES CONTAINING TRIDENTATE LIGANDS: A COMMENT P.S. BRATERMAN Department Received
of CRemist?y. 18 November
Glasgow
University,
Glasgow
GIZ SQQ.
1983; in final form 28 November
The changes observed in the lifetimes and polarizations in accord with the general localised excited-state model.
UK
1983
of ruthenium(U)
Crosby and co-workers [l] have reported changes in emission polarisation on cooling below 77 K in complexes of type [Ru(ll)(N-N-N)2] ?+, where N-N-N is a tridentate N-heterocyclic ligand. Their consequent analysis assigns the emitting states to 3E (in D&, split into B, + B2, E, A2 + A,, where the first and third pairs of states remain accidentally degenerate. We were immediately struck, however, by the resemblance between the three-state model required to describe the behaviour of the complexes in question and the empirical three-state model used by the same group in their now classical studies [2] of [Ru(bpy)3] 2+ and related species. The properties of [R~(bpy)~]~* can, however, be explained qualitatively and quantitatively by a model [3] in which the promoted electron of the emitting state is localised on one of the three ligands, in accord with the Raman [4] and electronic absorption [3] spectra of this state, and in which at some wavelengths this localisation is accomplished during, not after, the original excitation. Applying this principle to the molecules in question, they would be considered according to the selection rules for C&. Let the emitting states belong to some term 3I’, _This will span three out of the four irreducible representations of C&_ For example, if, plausibly, the metal configuration is bTb;a: in Czv, approximately e4b: in Dad, while the promoted electron occupies a &and orbital belonging to b,, the emitting states will belong to 3Bl, spanning Al i- A2 +B2 inch. This will give two mutually perpendicular 0 009-2614/84/S 03.00 0 Elsevier Science Publishers B-V_ (North-Holland Physics Publishing Division)
terpyridyl complexes as low temperature
are
components and one non-emitting component. So will any J?, f A?The compounds studied show a further unusual feature, which happily eliminates the possibility that the change in polarisation is due merely to excited-state localisation on cooling. Such a process could conceivably convert a high-temperature positive emission to a low-temperature negative emission, by transforming an E-absorber, E-emitter to an X- (oru-) absorber,y- (or x-) emitter; however, in this case. the change is from high-temperature negative emission to low-temperature positive emission, requiring a change and not merely a reduction in symmetry. Arguments of the same form can be applied 131 to species of type [Ru@py)L?] Z+ and also, with important modifications, to [Ru(l~py)~] ?+, as well as to related species. Thus, for these species, very low temperature photoselection emission data would be of the greatest interest. 1 thank Drs. GA_ Heath and L J. Yellowlees for useful discussions.
References [l]
ML. Stone and GA. Crosby, Chem. Phys. Letters 79 (1981) 169; S-F. Agnew, ML. Stone and GA. Crosby, Chem. Phys. Letters 85 (1982) 57.
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Volume 104, number 4
CHEMICAL PHYSICS LETTERS
[2] GD. Hager tid GA: Crosby, J. Am. Chem. Sot. 97 (1975)
7031; G-D. Hager, RJ. W&s and GA. Crosby, J. Am. Chem. sot.
97 (1975)
7037;
Gh. Crosby and W.H. Elfring Jr., J. Phys. Chem. 80 (1976) 2206. 131 P.S. Braterman,GA. Heath, A. Harriman and LJ. Yellowlees, J. Chem. Sot. Dalton Trans. (1983) 1801;
,406
10 February 1984
P.S. Braterman, GA. Heath and LJ. Yellowlees, in preparation. [4] R.F. Dallinger and WX Woodruff, J. Am. Chem. Sot. 101 (1979)
4391;
M. Fonter and RE. Hester,Chem. Phys. Letters 81 (1981) 42: P-G. Bradley, N. Kres:, B_A. Homberger, R.F. DaLtinger and WH. Woodruff, J. Am. Chem. Soi 103 (1981) 7441.