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

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

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).

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)

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)

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+ ?

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.

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).

Experimental Strategy - Photodissociation mass selection irradiation fragmentation & fragment analysis

† 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) 31938 - 31946

Experimental Setup Octopole Ion Guides TOF Accelerator Cold Paul Trap Quadrupole Bender Trap Injection Optics S. Xu, et al. PCCP 17 (2015) 31938 - 31946

† Experimental Setup electrospray needle desolvation capillary 10-1 mbar OPO + SHG/SFM YAG 220-2500 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) 31938 - 31946

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

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

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

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

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) 171102

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

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

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 600 570 540 510 480 450 420 Wavelength [nm] Bare ion +CH3CN Bulk Stockett and Brøndsted Nielsen, JCP 142 (2015) 171102

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): -1060 cm-1 (IV) and -1460 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

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

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

Low Energy MLCT Region of Ru(bpy)32+ in Vacuo TDDFT with spin-orbit interaction: (Heully et al., JCP 131 (2009) 184308) 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

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. 1000 – 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

Dramatis Personae NSF AMO PFC NSF Chemistry

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Ru(bpy)32+ Ru(bpy)32+·N2