Photochemistry of cis-tetracarbonylbis(trans-4-styrylpyridine)tungten(0): intraligand photoisomerization from upper excited states

The excited state reactivity and electronic absorption spectrum of cis-tetra-carbonylbis(trans-4-styrylpyridine)tungsten have been investigated. The lowest excited state in absorption is predominantly W→ trans-4-styrylpyridine charge transfer (CT) in character. An absorption band near 465 nm (ε{lunate} ≈ 10 000 1 mol^-1 cm^-1) in benzene is associated with this transition, and this band shifts to the blue in more polar or polarizable media in a manner comparable to other M→ ligand CT absorptions in metal carbonyls. Ligand field transitions can be observed in the vicinity of 400 nm, but the near-UV spectrum is dominated by intense intraligand. IL, π→π{black star} trans-4-styrylpyridine absorption. The photochemistry involves trans→cis isomerization of the coordinated ligand, but the wavelength dependence of the quantum yields indicates upper excited state reaction; the π→π{black star} IL is concluded to be responsible for the isomerization. The lowest excited CT state undergoes little or no photosubstitution or ligand isomerization. The trans→cis ligand isomerization quantum yield is <0.001 at 436 nm, but is ≈0.10 at 313 nm indicating relatively slow internal conversion from the IL to the CT state compared to trans→cis ligand reaction.