Ab initio methods are used to probe the underlying source of the red shift that arises in the absorption band of polymers that contain alternating phenyl and pyridine rings when the N atom of the latter is protonated. It is found that electronic charge transfers from the phenyl to the pyridine as an electron is excited from the HOMO to the LUMO in the protonated system but that no such transfer occurs if the N is unprotonated. Replacement of H atoms by cyano groups reinforces the idea that such an electron-withdrawing substituent facilitates the electron transfer when it is located on the pyridine ring, thereby stabilizing the excited state and thus enhancing the associated red shift; the opposite occurs when cyano is located on the phenyl ring. Unlike cyano and Cl, substitution by F and nitro groups leads to a less consistent pattern of absorption frequencies, suggesting that the effects of these substituents are not simply electrostatic in nature.