Raman spectroelectrochemical study of poly(N-methylaniline) at UV, blue, red, and NIR laser line excitations in solutions of different pH.

A detailed study on Raman spectroelectrochemistry of poly(N-methylaniline) (PNMA) layer deposited at a gold electrode was performed. Raman spectra were excited by four different laser wavelengths: UV line at 325 nm, a blue line at 442 nm, a red line at 633 nm, and a NIR line at 785 nm in solutions of different pH ranging from 1 to 9, and at different electrode potentials ranging from -0.5 V to 0.8 V. UV excitation reveals features characteristic for the reduced form of PNMA, even within the electrochemical potential range where oxidized forms of this polymer prevail. At a blue laser excitation, again, features of the reduced form are revealed, along with indications on the appearance of some kind of intermediate redox state within a definite potential window. Both red and NIR laser line excitations result in rich Raman features, disclosing all major redox forms as well as their interconversions by changing of electrode potential. The presence of polaronic form of PNMA even in pH-neutral and alkaline solutions has been disclosed. A detailed analysis of Raman vibrational bands is presented for different excitation wavelengths, different electrode potentials, and different solution acidities.