Transient Raman difference spectroscopy of nickel(II) uroporphyrin. pi. -. pi. complexes

A transient Raman difference spectrometer has been constructed and used to investigate the excited-state dynamics of nickel uroporphyrin I in {pi}-{pi} complexes with either 5-chloro-1,10-phenanthroline or 2,2{prime}-dimethyl-4,4{prime}-bipyridinium dichloride (methylviologen). The nature and lifetime of the nickel uroporphyrin excited state observed within the 10-ns laser pulses are like those exhibited by uncomplexed nickel porphyrins. However, we find that structure-sensitive Raman lines of the porphyrin are affected differently by complex formation in the excited state than in the ground state for the methylviologen complex. This result suggests that conformational changes, accompanying the creation of the metal-centered d-d excited state, modulate the geometry of the {pi}-{pi} complex. In particular, a conformational change from a ruffled, nonplanar geometry to a planar configuration appears to disrupt the unusual stacking interaction observed for the ground-state methylviologen complex.