Modern organic light emitting diode (OLED) devices are typically prepared in part by vapor phase deposition of a low work function metal cathode onto an organic electron carrier/ photoemitter. Tris(8-hydroxyquinolino)aluminum (Alq 3, 1) is a commonly used electron carrier/photoemitter, and it is reasonable to propose that chemical reduction of Alq 3 (with concomitant formation of metallic cations) can occur as a result of metal atom deposition, by analogy with well-known reduction processes of aromatic compounds which can be accomplished by activated or dissolved alkali and alkaline earth metals. Since the products of Alq3 reduction might vary in structure according to the deposited metal, the choice of cathodic metal may affect the performance of an OLED not only obviously, because of work function considerations, but also subtly, because of details of chemistry at the Alq3/cathode interface. It is not surprising, then, that analysis of possible chemical reactions between Alq 3 and several atomic alkali and alkaline earth metals (Li, K, Mg, and Ca) has been of considerable interest recently, both from theoretical and experimental perspectives. 1-4 Two items are noteworthy because of their absence from these reports. First, while they focus strongly on the expected shift tolower core level binding energies for the nitrogen atoms of the reduced quinolinate ligands, these studies either ignore any core level binding energy shifts for the oxygens of the ligands or suggest that such shifts are minor (and no mention is made of binding energy shifts for the central Al atom, either). Most surprising is that, while Mg (as Mg/Ag) is the most commonly used OLED contact with Alq 3, no experimental studies correlating the chemistry of Mg atom deposition on Alq 3 with spectroscopic observations have been reported. Since chemical reduction of polynuclear aromatic hydrocarbons with activated Mg can take a course quite different from that of Li, K, and Ca, the operationally significant Mg contact may in fact be structurally unrelated to these recently described models based on these other metals. We have now studied the deposition of this operationally key metal (and also of Al) onto thin films of Alq 3, and we find, by XPS and UPS analysis, a strong shift to higher binding core level energies occurs both at the ligand oxygens and at the central Al atom of the Alq3, which can be explained using simple organometallic models based on “solution”-derived observations. All procedures were carried out in an ultrahigh vacuum system consisting of three inter-connected chambers, which were used for surface preparation, film growth, and analysis, respectively. A 100 Å thick film of Alq3 was deposited on a gold substrate by thermal evaporation from an Al 2O3 crucible. Magnesium was then deposited incrementally on the Alq 3 film. XPS and UPS measurements were performed following each successive deposition event; the base pressure was maintained between 10 -8 and 10-10 Torr, for deposition and analysis, respectively. XPS data were collected using achromatic Al KR (1486.6 eV) or Zr Mú (151.4 eV) irradiation produced by a VSW double-anode source. Valence state data (UPS) were obtained using the He(I) ultraviolet emission (21.2 eV) line. 5 Changes in Mg(2p), C(1s), N(1s), O(1s), and Al(2p) core level binding energies (BE) were measured as a function of exposure of Alq 3 to Mg vapor. Analysis following initial Mg deposition (4 Å) showed the Mg(2p) peak to consist primarily of a high binding energy component (BE ) 51.6 eV), corresponding to Mg(II), and a smaller component at BE ) 49.3 eV. This latter peak grew with increasing exposure, and is assigned to metallic Mg. Peaks for C(1s), N(1s) and O(1s) all shifted by 0.6( 0.1 eV to higher BE after initial exposure to Mg, which is attributed to a shift of the whole molecular level structure of the organic film initially aligned with the Au substrate (see Table 1 in Supporting Information). While the peak shape and half-width for the C(1s) and Al(2p) peaks remained nearly constant regardless of Mg exposure, the N(1s) and O(1s) peak shapes changed considerably. Consistent with reports of K and Li deposition onto Alq3, reaction with Mg gave rise to a new, lower binding energy N(1s) peak component (experimental BE ) 399.1 eV; corrected BE) 398.5 eV; Figure 1). But in contrast, a new O(1s) peak component also appeared, which was shifted by 1.4 eV (corrected for molecular level realignment) to higher binding energy vs Alq 3 (experimental BE) 533.7 eV; corrected BE ) 533.1 eV; Figure 2). The Al(2p) signal also shifted to higher binding energy (experimental BE ) 75.5 eV; corrected BE) 74.9 eV). In agreement with previous measurements, 6,7 UPS measurements show that, while deposition o f 2 Å of Mg onto Alq3 attenuates spectral features, several peaks remain identifiable and are shifted (by realignment of molecular levels) to higher binding energies by∼0.6 eV. A new state, at approximately 1.5 eV above the (corrected) HOMO of Alq 3, was also detected † Department of Electrical Engineering. ‡ Department of Chemistry. (1) Choong, V. E.; Mason, M. G.; Tang, C. W.; Gao, Y. Appl. Phys. Lett. 1998, 72, 2689. (2) Zhang, R. Q.; Hou, X. Y.; Lee, S. T. Appl. Phys. Lett. 1999, 74, 1612. (3) Johanson, N.; Osada, T.; Stafstro ̈m, S.; Salaneck, W. R.; Parente, V.; dos Santos, D. A.; Crispin, X.; Bre ́das, J. L.J. Chem. Phys. 1999, 111, 21572163. (4) Curioni, A.; Andreoni, W.J. Am. Chem. Soc. 1999, 121, 8216-8220. (5) XPS and UPS spectra were collected with a Perkin-Elmer double-pass cylindrical mirror analyzer. Resolution for the UPS and XPS measurements were 0.15 and 0.7 eV, respectively. Energy scales were calibrated using the Fermi edge of a clean gold surface and the Mg(2p) core level of a freshly deposited Mg layer. Al KR irradiation was used to collect O(1s), C(1s), and N(1s) data, and Zr Mú irradiation was used for Mg(2p) and Al(2p) measurements. Figure 1. N(1s) XPS spectra of Alq 3 with increasing Mg exposure. 5391 J. Am. Chem. Soc. 2000,122,5391-5392