Manifestation of Peripheral NO2-Substitution in the Resonance Raman Scattering Spectra of Tetraarylporphyrins

ISSN： 0021-9037

Source： Journal of Applied Spectroscopy, Vol.67, Iss.5, 2000-09, pp. : 796-805

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Abstract

The resonance Raman scattering (RS) spectra of a series of nitro-substituted derivatives of the free base of tetraphenyl porphyrin that were obtained on continuous excitation near the Soret band have been investigated. For the molecules in which the NO_2 group is located in the &bgr;-position of pyrrole rings or in the para-position of meso-phenyl substituents, an intense line of the mode within the range 1340–1355 cm^−1 is discovered. Based on the obtained resonance RS spectra of the diprotonated derivatives of nitro-tetraarylporphyrins, it is assigned to symmetrical vibration of a nitro group (&ngr;_sNO_2). Activation of the &ngr;_sNO_2 vibration is indicative of the existence of the &pgr;-delocalization effect for a number of nitro-substituted porphyrins at which interaction between the molecular orbitals of the nitro-substituents and the porphyrin macrocycle occurs. Moreover, the &pgr;-delocalization is typical of both direct addition of a nitro-substituent to the porphyrin ring and of its binding via a meso-phenyl group. The results obtained demonstrate the informative value of the resonance RS spectroscopy for creating and studying model donor–acceptor systems with the participation of porphyrins.