Nature of the improved Raman scattering from α-copper phthalocyanine particles on a compacted electrode

Abstract

Electrodes consisting of a compacted mixture of powders of silver metal and α-copper phthalocyanine (CuPc) have been used to study small pigment particles bonded to the metal surface using resonance Raman spectroscopy as a probe of in situ changes. This system provides a different starting point for the study of the nature of the terms which give rise to improved Raman scattering at metal surfaces. An understanding of these processes is vital if Raman spectroscopy is to achieve its potential as an in situ method for the detection of reactions at metal/organic interfaces in aqueous solution. Resonance excitation profiles (REPs) indicate that the improved scattering is in the region of the phthalocyanine absorption (Q band) and resembles a CuPc multilayer. There is a significant electronic interaction between the particles and the metal surface with a downward shift in the 0–0 transition energy of the Q band of ca. 30 nm. The REPs from the electrode surface are more clearly resolved than for CuPc itself. There are fewer effective vibronic contributions especially at negative potentials and scattering is greater from higher vibronic levels. New bands not present in the spectrum of CuPc appear at negative potentials and the appearance coincides with changes in intensity of bands observed at all potentials. Two different mechanisms of enhancement are indicated: a resonance mechanism where increased scattering efficiency is achieved both by good sample presentation and by an electronic interaction between the particles and the metal, and a surface-enhanced resonance effect which competes with the resonance mechanism at negative potentials. Both the resonance effect and the surface-enhanced effect give rise to REPs attributable to phthalocyanine Q-band interactions. Both processes arise from a long-range ‘charge-transfer’ mechanisms between the CuPc particles and the metal rather than from short-range metal–monolayer interactions.