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 π-π complexes with either 5-chloro-l,10-phenanthroline or 2,2′-dimethyl-4,4′-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 π-π 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. © 1990 American Chemical Society FTIR Study of Vibrational Relaxation in KCl04 Crystal R. Bini, P. Foggi, P. R. Salvi, and V. Schettino*Laboratorio di Spettroscopia Molecolare, Dipartimento di Chimica, Universita' di Firenze. Via G.Capponi 9,*To whom correspondence should be addressed. The infrared spectrum of KClO4 single crystal has been studied in the region of weak infrared v1, v2, and v1 + 2v2 vibrational excitons at low temperature. The v2 dispersion is found to be ≃ 10 cm -1. The infrared and Raman spectroscopy in the v1 region, in close coincidence with the first overtone of v2, is discussed in terms of Fermi resonance in crystals. The dependence on temperature of the bandwidths of the v1 and v1 + 2v2 modes has been measured in the 20-160 K temperature range. Our bandwidth data are interpreted on the basis of current theories on relaxation mechanisms. In particular, the main decay processes in the v1 and v1, + 2v2 case involve the cubic anharmonicity and are determined only by depopulation. The calculated decays compare well with experimental results. On the whole, our decay data are also in agreement with recently reported picosecond coherent anti-Stokes Raman spectroscopy (CARS) bandwidth measurements. © 1990 American Chemical Society.