Electron Affinities of 1,1-Diaryl-2,3,4,5-tetraphenylsiloles: Direct Measurements and Comparison with Experimental and Theoretical Estimates Xiaowei Zhan, ² Chad Risko, ² Fabrice Amy, ‡ Calvin Chan, ‡ Wei Zhao, ‡ Stephen Barlow, ² Antoine Kahn,* ,‡ Jean-Luc Bre ´ das,* ,² and Seth R. Marder* ,² Contribution from the School of Chemistry and Biochemistry and the Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, and Department of Electrical Engineering, Princeton UniVersity, Princeton, New Jersey 08544 Received February 22, 2005; E-mail: seth.marder@chemistry.gatech.edu Abstract: We present a comprehensive experimental and theoretical characterization of the electronic structure of four 1,1-diaryl-2,3,4,5-tetraphenylsiloles (aryl ) phenyl, 2-(9,9-dimethylfluorenyl), 2-thienyl, pentafluorophenyl). Solid-state electron affinities and ionization potentials of these siloles were measured using inverse-photoelectron spectroscopy (IPES) and photoelectron spectroscopy (PES), respectively; the density of electronic states obtained from calculations performed at the density functional theory (DFT) level corresponds very well to the PES and IPES data. The direct IPES measurements of electron affinity were then used to assess alternative estimates based on electrochemical and/or optical data. We also used DFT to calculate the reorganization energies for the electron-transfer reactions between these siloles and their radical anions. Additionally, optical data and ionization potential and electron affinity data were utilized to estimate the binding energies of excitons in these siloles. I. Introduction Siloles (silacyclopentadienes, Figure 1) have recently attracted attention as useful materials for organic electronics since they may exhibit high electron mobilities and high photolumines- cence quantum yields. Time-of-flight electron mobilities of 1 were found to be ca. 2 × 10 -4 cm 2 /Vs, a 2-order of magnitude improvement compared to the well-established electron-transport (ET) material tris(8-hydroquinolinato)aluminum (Alq 3 ); more- over, the electron transport was shown to be nondispersive and independent of exposure to air. 1-3 Accordingly, siloles have been used as efficient ET materials in organic light-emitting diodes (OLEDs). 4 Furthermore, 2 possesses a photoluminescence (PL) quantum yield of 97 ( 3% in the solid state and has been used as an effective lumophore in OLEDs; for example, an OLED using 2 as an emissive layer and 1 as an ET layer shows a very low operating voltage of 2.5 V and a high external electroluminescence (EL) quantum efficiency of 4.8%. 1 Another EL device based on 3 shows an excellent external EL quantum efficiency of 8%, 5 while 4 has been shown to form a highly emissive exciplex (PL quantum yield of 62%) at the interface with the hole-transport material N,N′-diphenyl-N,N′-bis(1-naph- thalenyl)-(1,1′-biphenyl)-4,4′-diamine (NPD) and has been exploited in a three-layer device with the configuration Ag: Mg/1/4/NPD/ITO, which has an external EL quantum efficiency of 3.4%. 6 In designing organic electronic devices, it is important to be able to assess hole- and electron-injection barriers at the various organic/organic and organic/inorganic interfaces. These are typically estimated by comparing ionization potentials (IPs) and electron affinities (EAs) of the molecular components with each other and/or with the Fermi energies of inorganic electrode materials, respectively. 7 Although approximate IPs and EAs have often been derived from solution electrochemical data, such evaluations require reversible electrochemistry and assume similar solvation effects and solid-state polarization effects on the formation of ions in both the species of interest and the reference compound. The most direct experimental probes of IP and EA are photoelectron spectroscopy (PES) and inverse- photoelectron spectroscopy (IPES), respectively. Here we report a study of the electronic properties of 1,1- diaryl-2,3,4,5-tetraphenylsiloles. We selected this class of siloles given that: (i) it has previously been reported that OLEDs based on siloles with R 1 ) R 1 ′ ) Ph are much brighter and more ² Georgia Institute of Technology. ‡ Princeton University. (1) Murata, H.; Kafafi, Z. H.; Uchida, M. Appl. Phys. Lett. 2002, 80, 189- 191. (2) Murata, H.; Malliaras, G. G.; Uchida, M.; Shen, Y.; Kafafi, Z. H. Chem. Phys. Lett. 2001, 339, 161-166. (3) Ma ¨kinen, A. J.; Uchida, M.; Kafafi, Z. H. J. Appl. Phys. 2004, 95, 2832- 2838. (4) Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi, S. J. Am. Chem. Soc. 1996, 118, 11974-11975. (5) Chen, H. Y.; Lam, W. Y.; Luo, J. D.; Ho, Y. L.; Tang, B. Z.; Zhu, D. B.; Wong, M.; Kwok, H. S. Appl. Phys. Lett. 2002, 81, 574-576. (6) Palilis, L. C.; Ma ¨kinen, A. J.; Uchida, M.; Kafafi, Z. H. Appl. Phys. Lett. 2003, 82, 2209-2211. (7) This approach overlooks effects such as vacuum-level offsets brought about due to interface dipole formation and effects due to creation of new species with “gap states” close to the interfaces. Indeed, Kafafi and co-workers have found gap states formed at the silole/magnesium interface using PES. However, these effects are likely to be negligible in organic/organic interfaces and will be very material-specific in the case of organic/inorganic interfaces; see ref 3. Published on Web 06/04/2005 10.1021/ja051139i CCC: $30.25 © 2005 American Chemical Society J. AM. CHEM. SOC. 2005, 127, 9021-9029 9 9021