Resonance Raman Spectra of 4-(2-Pyridylazo)-resorcinol (PAR) and of its Cu(II) and Zn(II)Chelates: the Nature of the Low-Energy Electronic Transition

Abstract

The long-standing question of the nature of the low-energy transition present in the optical spectrum of azo dyes was addressed by investigating the resonance Raman spectrum of 4-(2-pyridylazo)resorcinol (PAR). The Raman spectra were obtained at different pH values where the prevailing species are the monoanionic (HL-) or the dianionic (L2-) forms. The excitation profiles and absorption spectra were calculated satisfactorily using a single excited state as responsible for the low-energy transition present in the optical spectrum. The remarkably stable Cu(II) and Zn(II) chelates involving the coordination of the HL- species to the metal ions were also investigated, and again the excitation profiles and absorption spectra were calculated satisfactorily using a single excited state. In fact, the concerted use of the transform method and time-dependent theory indicates that the displacement parameters for the metal chelates are much more similar to those obtained for the L2- species than those for HL- species. In the case of the Cu(II) chelate, a long progression of overtones and combination bands was observed, which is uncommon for such an extended chromophore. Concerning the nature of the electronic transition responsible for the resonance enhancement, the extensive electronic delocalization of the chromophore and the complex composition of the normal modes preferentially enhanced must be noted. In other words, it is expected that the ππ* and nπ* states are thoroughly mixed in this class of the molecular systems. © 1997 by John Wiley & Sons, Ltd.