X-ray photoelectron spectroscopy as a probe of intermolecular interactionsin porphyrin polymer thin films

High-resolution X-ray photoelectron spectroscopy (XPS) has been applied to a series of free-base and Zn-porphyrin polymers in which the macrocycles ar e separated by oligo(phenylene vinylene) units (OPV) of different, controll able lengths. Neat films of all the Zn-porphyrin polymers unexpectedly reve al two peaks in the N Is XPS region (similar to 400 eV). The peak areas var y with the length of the OPV bridge, suggesting an intermolecular interacti on between the porphyrin and linker subunits. A series of XPS analyses were performed to identify the different interactions taking place in these thi n films. To inhibit interpolymer interactions, one of the polymers was inco rporated into a nonconjugated PMMA matrix, collapsing the N1s spectrum to a single peak at 398.0 eV, relative to the neat film signals at 397.8 and 40 0.1 eV. In a conjugated matrix of OPV, two peaks remain at 401.7 and 399.5 eV. Extensive vacuum drying of the neat film leads to a single peak at 398. 3 eV, suggesting loss of trapped solvent molecules. Ultimately, we attribut e the lower energy signal of the neat films to solvent ligation, and the hi gher energy peak is attributed to interactions between the porphyrins and c onjugated bridges on nearby polymer chains. This interpretation is successf ully applied to the N 1s XPS data from a previously reported Zn-porphyrin o ligomer-based multilayer array.