Photophysics of Mononuclear Cu(I) Complexes

1. Photophysics of Mononuclear Cu(I) Complexes Alan Gabrielli* and C. David Sherrill Center for Computational Molecular Science and Technology School of Chemistry and Biochemistry *Southern Polytechnic State University

2. Background • Some complexes of copper(I) are luminescent • Singlet Cu(I) complexes are excited to a singlet excited state • The singlet ES relaxes to a triplet ES via a spin-forbidden transition, accompanied by a structural change • The ES triplet relaxes to the GS singlet (with distorted geometry via Franck-Condon Principle)

3. Photoluminescence of Cu complex

4. Theoretical methods and basis sets • Theoretical methods: Hartree-Fock and CIS for geometries, CIS and TDDFT for excitation energies • Basis sets used: • 6-31G, 6-31G* • ES calculations are done single point at the GS geometry and then are optimized for the first ES triplet geometry • ∆E(fluor) is taken from the GS singlet energy (at the ES triplet geometry) to the first ES triplet energy

5. Excitation of Cu(S-t-Bu)-2 • GS singlet Cu(I) is linear • ES triplet is bent, a characteristic of Cu(II), implying metal-to-ligand charge transfer has occured

6. Calculated vs. Observed Emission for Cu(S-t-Bu)-2 (B3LYP 6-31+G*)

7. Copper(I) complexes of phenanthroline (phen) derivatives Cu(phen)2 Cu(1,9-diphenylphen)2 Cu(1,9-dimethylphen)2

8. Copper(I) complexes of phenanthroline (phen) derivatives • 1,10-phenanthrolines bind strongly to Cu(I) • Cu(I)(phen)2+ is not photoemissive • Bulky substituents exhibit stronger and longer photoluminescence • Metal-to-ligand charge transfer yields Cu(II), which prefers square-planar over tetrahedral geometry • Ligands which obstruct a shift to a square-planar conformer should favor stronger photoluminescence

9. HF and CIS 6-31G* results

10. Electron-density maps for T1 State of Cu(phen)2+ Electron detach density Electron attach density The lowest-lying triplet state is a p - p * transition

11. Cu(phen)2+ and derivatives • The optimized triplet geometries are largely unchanged from the GS singlet • Cu atomic charges unchanged • Metal-to-ligand charge transfer not supported by these results

12. Sulfide derivatives of diphenylphenanthroline • Ortho substitution of -CH2-S-R to phenyl groups of 2,9-diphenylphenanthroline yields new tetradentate ligands • Cu(I) complexes of these ligands are potentially strongly photoluminescent

13. R = ethyl • Relative freedom of sulfide ligands should allow greater range of motion

14. X-ray structure In the observed structure, one S has the -S conformation and one S has the -R conformation * C. Fahrni, unpublished results

15. Luminescence Emission Spectrum lmax (observed) ≈ 625 nm * C. Fahrni, unpublished results

16. HF and CIS Results at 6-31G*

17. Comparison of Experimental and Calculated Geometries

18. Conclusions • Calculations on Cu complexes give a reasonable degree of accuracy even at the HF 6-31G* level. • The first triplet states of Cu complexes of phenanthroline and substituted phenanthrolines are ligand-ligand charge transfers, not metal-ligand charge transfers as had been previously supposed. • Calculations at higher levels of theory are in progress.