RESONANCE RAMAN-SPECTRA OF 4(2-PYRIDYLAZO)RESORCINOL (PAR) AND OF ITSCU(II) AND ZN(II) CHELATES - THE NATURE OF THE LOW-ENERGY ELECTRONIC-TRANSITION

The long-standing question of the nature of the low-energy transition present in the optical spectrum of azo dyes was addressed by investiga ting the resonance Raman spectrum of 4-(2-pyridylazo)resorcinol (PAR). The Raman spectra were obtained at different pH values where the prev ailing species are the monoanionic (HL(-)) or the dianionic (L(2-)) fo rms. The excitation profiles and absorption spectra were calculated sa tisfactorily using a single excited state as responsible for the low-e nergy transition present in the optical spectrum. The remarkably stabl e Cu(II) and Zn(II) chelates involving the coordination of the HL(-) s pecies to the metal ions were also investigated, and again the excitat ion profiles and absorption spectra were calculated satisfactorily usi ng a single excited state. In fact, the concerted use of the transform method and time-dependent theory indicates that the displacement para meters for the metal chelates are much more similar to those obtained for the L(2-) 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. Con cerning the nature of the electronic transition responsible for the re sonance enhancement, the extensive electronic delocalization of the ch romophore and the complex composition of the normal modes preferential ly enhanced must be noted. In other words, it is expected that the pi pi and n pi* states are thoroughly mixed in this class of the molecul ar systems. (C) 1997 by John Wiley & Sons, Ltd.