The unique carbon cluster Cmshows remarkable photophys-ical properties. 1-7 Laser excitation generates the excited singlet state, which undergoes very efficient intersystem crossing16· 4’7 to a long-lived triplet state. The lifetime of the singletstate was reported initially as 33 ps, 3 but more recent measurements provide values near 1 ns. 4-6 The triplet-state lifetime has been reported variously as 40# tsla and> 280 ps5 in solution and as ca. 300 ps in a toluene glass. 3 It is of interest to examine how these relatively long excited-state lifetimes are affected in derivatives of Ceo, in particular, in 1: 1 complexes with transition metal centers. 8 In this paper we report the first measurements on the formation and lifetime of an electronic excited state of a metalated Ceo derivative. The indenyliridium complex (7? 5-C9H7) Ir (CO)(i? 2-Ceo), 8a [-Cm], forms air-stable green solutions in noncoordinating organic solvents. The UV features of the electronic absorption spectrum of [Ir-Ceo] are little changed from those of Ceo itself; however, the visible spectrum shows a well-defined new absorption band at 436 nm. 8a The position of this band does not change with solvent polarity (hexane, benzene, dichloromethane) or upon the addition of an electron to the Cm ligand. 9 These characteristics suggest that the 436-nmband is not due to an IMo-Cm charge-transfer transition but rather arises from a transition largely centered on the Coo ligand. One possibility is a relatively localized ir-* type transition involving the coordinated double bond, a transition derived from the HOMO-LUMO (h „-* tju) transitions that are symmetry forbidden in free Coo- In this regard, it is significant that a similar new band is seen in the recently reported spectrum of C6oO, 10 The transient absorption spectrum of [Ir-Coo] shown in Figure

1 was recorded 50 ns after pulsed 388-nm irradiation of a nitrogenpurged toluene solution. 11 Three strong transient absorptions are observed at 420, 490, and 720 nm, very similar to those observed for Cm itself. 5· 6· 12 The same transient spectrum is obtained for [Ir-Ceo] following excitation at 308 nm. The transient absorptions formed by laser irradiation decay to> 90%,(1)(a) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez,. M.; Anz, S. J.; Whetten, R. L. J. Phys. Chem. 1991, 95, 12.(b) Arbogast, J. W.; Foote, CS; Kao, M. J. Am. Chem. Soc. 1992, 114, 2277.(2)(a) Tutt, L. W.; Kost, A. Nature 1992, 356,225.(b) Wang, Y.; Cheng, L.-T. J. Phys. Chem. 1992, 96, 1530.(c) Kafafi, Z. H.; Lindle, J. R.;