Photoelectron Spectroscopy

1. Photoelectron Spectroscopy • Lecture 10: Gas phase vs solid/solution phase measurements • Gas phase vs solid phase UPS • Gas phase ionization vs solution phase oxidation

2. Why gas phase spectroscopy? • Allows us to study isolated molecules. • This is the way we usually visualize electronic structure of molecules. • We of course know that solution, environment is a factor, but how big a factor? • Need to consider relative strength of environmental factors compared to intramolecular forces.

3. M+ ΔEsolvation (M+) + + - + - - + + - - + - + + M+solv IE (M) IE (Msolv) ΔEsolvation (M) Msolv Solvation Effects Gas phase Condensed phase M

4. O C O C R3 PMe3 PPh3 2064.1 cm-1 2068.9 cm-1 C O PR3-H+ PR3 +H+ PMe3 PPh3 7.85 3.05 Measuring the donor strength of phosphines Solution-phase IR (νCO) of Ni(CO)3L1 … … and the pKa of the conjugate acid (PR3H+) in aqueous solution.2 PMe3 is a better donor than PPh3 (1) Tolman, T.A. J. Am. Chem. Soc. 1970, 92, 2953. (2) Shaw, B.L. J. Chem. Soc., Chem. Comm. 1979, 104.

5. PR3 + H+ PR3H+ PMe3 PPh3 PMe3 PPh3 223.5 kcal/mol 226.7 kcal/mol 8.57 eV 7.83 eV Measuring the donor strength of phosphines Gas-phase proton affinities1…. …and gas-phase PES studies.2 - 0.74 eV PPh3 is a better donor than PMe3 Trend in donor strength is reversed with different phases. (1) Ikuta, S., et al. J. Am. Chem. Soc. 1982, 104, 5899. (2) Bancroft, G.M., et al. Inorg. Chem. 1986, 25, 3675.

6. Au d Bare Au Au Fermi 4 A 16 A 50 A 100 A Au substrate 16 12 8 4 Ionization Energy (eV) Preparation of thin films Condensed phase samples were prepared by vapor deposition… Mo(CO)4dppe …with an optimum film thickness of 50-150 A.

7. 0 kinetic energy Photoionization of a Solid: What has to happen for a photoelectron to escape? 3) Penetration through the surface 2) Transport to the surface • Photoexitation • of the electron EVB Evac  EF EVB Valence Band vacuum bulk surface

8. Escape Depths: The Universal Curve

9. Me Me P Me P Me OC CO Mo OC CO Ph Ph Ph P P Ph OC CO Mo OC CO Comparison of dmpe and dppe substitution in the gas and condensed phase Gas phase 1.1 eV Condensed phase dmpe 0.2 eV dppe 10 9 8 7 6 5 Ionization Energy (eV)

10. Electrochemistry • Oxidation: also removal of an electron: • M  M+ + e- • Solvation effects. • Timescale is much slower than photoionization, allows geometry changes to occur. • Measure an adiabatic value, not vertical. • Reference is reference electrode, or internal reference such as ferrocene oxidation. • Different definition of second ionization/oxidation.

11. Relationship Between Ionization and Oxidation Fc Fc+ + e- E(V) Fc Fc+ + e-DG DG = -nFE

12. Correlation of Ionization and Oxidation Energies for Substituted Anthracenes 2.2 2 1.8 1.6 1.4 E =0.86(IE)-4.91 1/2 1.2 2 R =0.97 1 7 7.2 7.4 7.6 7.8 8 8.2 8.4 Ionization Energy (eV) Masnovi et al., Can. J. Chem.1984, 62, 2552.

13. 0.8 0.7 E1/2 vs Ag/AgNO3 (CH3CN) 0.6 0.5 y = 0.0373x + 0.3259 R2 = 0.02 0.4 7.4 7.6 7.8 8 8.2 Ionization Energy (eV) Correlation of Ionization and Oxidation Energies for 1,2-Dithiins E-C Mechanism: R. S. Glass et al. JACS 2000, 122, 5052-5064

14. Summary • Important to think about environmental effects. • Gas phase and solid/solution phase measurements do not always show the same trends.