1. Electronic Spectrum of Cryogenic Ruthenium-Tris-Bipyridine Dications
Shuang Xu, James E. T. Smith and J. Mathias Weber
May 9, 2016

2. Motivation Ru-polypyridine complexes:
Important class of molecules with many applications:
Catalysis
Light harvesting
Staining …

3. Motivation Ru-polypyridine complexes:
Important class of molecules with many applications.
Ru(bpy)32+ (D3 symmetry) is the most prototypical complex in this group (bpy = 2,2’-bipyridine).

4. Motivation Ru-polypyridine complexes:
“Old” problem (first spectrum in 1966), but experimental spectra in solution and crystals are broad. (Palmer & Piper, Inorg. Chem. 5 (1966) 864, Noyes Laboratory, UI)

5. Motivation Ru-polypyridine complexes:
“Old” problem (first spectrum in 1966), but experimental spectra in solution and crystals are broad. (Palmer & Piper, Inorg. Chem. 5 (1966) 864, Noyes Laboratory, UI)
Low T spectra sharper, but shifted compared to solution spectra (Felix et al., JACS 102 (1980) 4096)

6. Motivation Ru-polypyridine complexes:
“Old” problem (first spectrum in 1966), but experimental spectra in solution and crystals are broad. (Palmer & Piper, Inorg. Chem. 5 (1966) 864, Noyes Laboratory, UI)
Low T spectra sharper, but shifted compared to solution spectra
What is the intrinsic absorption spectrum of Ru(bpy)32+ ?

7. Motivation Ru-polypyridine complexes:
“Old” problem (first spectrum in 1966), but experimental spectra in solution and crystals are broad. (Palmer & Piper, Inorg. Chem. 5 (1966) 864, Noyes Laboratory, UI)
Low T spectra sharper, but shifted compared to solution spectra
What is the intrinsic absorption spectrum of Ru(bpy)32+?  Need the best possible experimental spectrum for detailed understanding and to benchmark theory on excited states.

8. Motivation Ru-polypyridine complexes:
“Old” problem (first spectrum in 1966), but experimental spectra in solution and crystals are broad. (Palmer & Piper, Inorg. Chem. 5 (1966) 864, Noyes Laboratory, UI)
Low T spectra sharper, but shifted compared to solution spectra
What is the intrinsic absorption spectrum of Ru(bpy)32+?
Cryogenic spectroscopy of Ru(bpy)32+ in vacuo:
No solvent influence: measure intrinsic spectrum, get solvatochromic shifts.
Low temperature: suppress hot bands to reduce spectral congestion.
Direct comparison with calculations (without solvent).

9. Experimental Strategy - Photodissociation
mass selection
irradiation
fragmentation & fragment analysis

10. † Experimental Setup electrospray needle desolvation capillary
mass gate
ion optics
ejection/acceleration
optics
quadrupole
bender
octopole
guides
desolvation
capillary
MCP
reflectron †
electrospray needle
10-1 mbar
OPO + SHG/SFM
YAG
10-4 mbar
10-6 mbar
10-8 mbar
cryogenic ion trap
power meter
S. Xu, et al. PCCP 17 (2015)

11. Experimental Setup Octopole Ion Guides TOF Accelerator Cold Paul Trap
Quadrupole
Bender
Trap Injection
Optics
S. Xu, et al. PCCP 17 (2015)

12. † Experimental Setup electrospray needle desolvation capillary
10-1 mbar
OPO + SHG/SFM
YAG
nm  7 ns pulses  5 cm-1 bandwidth
10-4 mbar
octopole
guides
mass gate
10-6 mbar
ion optics
ion trajectory in flight tube
quadrupole
bender
10-8 mbar
cryogenic ion trap
ejection/acceleration
optics
MCP
power meter
reflectron
S. Xu, et al. PCCP 17 (2015)

13. Experimental Strategy - Photodissociation
Problem: Ru(bpy)32+  Ru(bpy)22+ + bpy;
 E  4 eV
S. Xu, J. E. T. Smith, JMW, submitted

14. Experimental Strategy - Photodissociation
Problem: Ru(bpy)32+  Ru(bpy)22+ + bpy;
 E  4 eV
Solution: Use N2 adducts as “messengers” Ru(bpy)32+ · N2  Ru(bpy)32+ + N2;
 E  50 meV
S. Xu, J. E. T. Smith, JMW, submitted

15. UV/vis Spectrum of Ru(bpy)32+ in Solution
Metal-to-
Ligand
Charge
Transfer
MLCT
S. Xu, J. E. T. Smith, JMW, submitted

16. UV/vis Spectrum of Ru(bpy)32+ in Solution
p-p*
p-p*
MLCT
S. Xu, J. E. T. Smith, JMW, submitted

17. UV/vis Spectrum of Ru(bpy)32+ in Vacuo
Previous work at room temperature:
Kirketerp and Brøndsted Nielsen, IJMS 297 (2010) 63
Stockett and Brøndsted Nielsen, JCP 142 (2015)

18. UV/vis Spectrum of Ru(bpy)32+ in Vacuo
Cryogenic spectrum:
Several partially resolved electronic bands
Still broad
S. Xu, J. E. T. Smith, JMW, submitted

19. UV/vis Spectrum of Ru(bpy)32+ in Vacuo
Selection rules for electronic excitations of a complex with D3 symmetry: A2 E
(simple TDDFT calculations, B3LYP, def2-TZVP)
Assignments:
Bands I – VIII: MLCT
Band IX: p-p*
Zoom rectangle
S. Xu, J. E. T. Smith, JMW, submitted

20. Low Energy MLCT Region of Ru(bpy)32+ in Vacuo
Previous work at room temperature:
Low energy MLCT band unaffected by single solvent molecules
Some substructure, unclear origin
Wavelength [nm]
Bare ion
+CH3CN
Bulk
Stockett and Brøndsted Nielsen, JCP 142 (2015)

21. Low Energy MLCT Region of Ru(bpy)32+ in Vacuo
solution
At 25 K trap temperature, N2 tagging:
Suppression of hot bands
Clearly resolvable substructure
Solvatochromic shifts of main peaks (aq): cm-1 (IV) and cm-1 (V)
No vibrational features
Individual electronic bands are still broad
Nielsen, RT
wavenumber [103 cm-1]
S. Xu, J. E. T. Smith, JMW, submitted
Residual width due to lifetime broadening: t(MLCT) < 30 fs (ultrafast intersystem crossing!)
Yoon et al., Mol. Phys. 104 (2006) 1275

22. Low Energy MLCT Region of Ru(bpy)32+ in Vacuo
TDDFT without spin-orbit interaction:
Qualitative description of most intense bands (comp. scale shifted by +800 cm-1).
S. Xu, J. E. T. Smith, JMW, submitted

23. Low Energy MLCT Region of Ru(bpy)32+ in Vacuo
TDDFT without spin-orbit interaction:
Qualitative description of most intense bands (comp. scale shifted by +800 cm-1).
Onset of spectrum not explained.
Relative intensities not reliable.
Ultrafast ISC suggests that spin-orbit interaction is important!
S. Xu, J. E. T. Smith, JMW, submitted

24. Low Energy MLCT Region of Ru(bpy)32+ in Vacuo
TDDFT with spin-orbit interaction:
(Heully et al., JCP 131 (2009) )
Qualitative description of most intense bands (scale shifted by +800 cm-1).
Onset of spectrum well recovered.
Relative intensities not reliable.
S. Xu, J. E. T. Smith, JMW, submitted

25. Summary
Cryogenic ion spectroscopy allows detailed assignment of electronic states within MLCT band manifold of Ru(bpy)32+.
Lowest singlet MLCT state is at (18.5 ± 0.2)·103 cm-1.
Spin-orbit interaction is essential to recover low energy bands.
Solvatochromic shifts: ca – 1500 cm-1
Additional metal-tris-bipyridine  see talk TD 04 by Shuang Xu
Solvatochromic effects in Ru complexes: see talk TD 06 by James E.T. Smith

26. Dramatis Personae
NSF AMO
PFC
NSF Chemistry

27. Thank you for your attention

28. Ru(bpy)32+ Ru(bpy)32+·N2