X-ray photoelectron spectroscopy as a probe of intermolecular interactions in 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 are separated by oligo(phenylene vinylene) units (OPV) of different, controllable lengths. Neat films of all the Zn-porphyrin polymers unexpectedly reveal two peaks in the N Is XPS region (similar to 400 eV). The peak areas vary with the length of the OPV bridge, suggesting an intermolecular interaction between the porphyrin and linker subunits. A series of XPS analyses were performed to identify the different interactions taking place in these thin films. To inhibit interpolymer interactions, one of the polymers was incorporated 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 400.1 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 attribute the lower energy signal of the neat films to solvent ligation, and the higher energy peak is attributed to interactions between the porphyrins and conjugated bridges on nearby polymer chains. This interpretation is successfully applied to the N 1s XPS data from a previously reported Zn-porphyrin oligomer-based multilayer array.